V. SUPPLEMENTARY PAPERS

1. Field water balance in cropped lucerne plots
2. Some physical properties of sandy soils and sand dunes in Iraq
3. The sandy soils of the Kingdom of Saudi Arabia

1. Field water balance in cropped lucerne plots

by
V. D. Krentos, Y. Stylianou and Ch. Metochis
Agricultural Research Institute, Nicosia, Cyprus

SUMMARY

The instantaneous profile method was applied in successively cropped and fallow (covered) plots in order to assess the drainage component in a field water balance study.

Moisture changes in a layered medium textured soil were frequently monitored with a neutron moisture gauge with concurrent measurements of matric suction by mercury tensiometers placed at 30 cm intervals up to a depth of 240 cm. From these data it was possible to determine the unsaturated hydraulic conductivity of the draining fallow (covered) plot over a period of 100 days from which the drainage component occurring over a series of crop-irrigation cycles over a whole year was calculated. Thus the actual evapotranspiration of lucerne was determined from soil moisture changes adjusted for drainage.

Yields of dry matter, changes in soil moisture, drainage and evapotranspiration values during each crop cycle are presented and discussed. Actual evapotranspiration is compared with potential evapotranspiration (Penman) and pan evaporation and it is concluded that this relatively simple procedure provides a realistic estimate of crop water requirements and may be adopted for water use efficiency studies.

4.1 INTRODUCTION

Irrigated agriculture in arid and semi-arid regions is of vital importance to their national economies. It is not uncommon that the increase in agricultural productivity is not restricted by the availability of land resources but rather by the very limitation imposed by low rainfall and the short supply of good quality irrigation water (Fried and Barrada, 1967). This is particularly true for countries like Cyprus where irrigated crops, representing only 13 percent of the cultivated land but contributing over 50 percent of the total value for agricultural production, impose a severe burden on the rapidly depleting underground water resources of the island.

Thus the critical question poised starkly over Cyprus is whether the remaining balance of its water crop can indeed sustain the requirements of existing plantations excluding any thought, for the moment at least, of expanding irrigated agriculture. This need for "survival", therefore, confers a new dimension on the necessity for the most efficient use of its water resources. To be sure, it requires as precise a knowledge of the optimum crop water requirements as could possibly be acquired.

Field studies on crop water requirements carried out in Cyprus prior to 1971, such the one presently reported (Stylianou and Krentos, 1972), were pervaded, as elsewhere, by the prevailing concept of "field capacity" with its consequent confines on the thinking of soil moisture availability. Gardner (1966 and 1967) has pointed that this concept, based on the assumption that no downward movement of soil water occurred at water contents below this upper limit, disregards the physical processes known to take place continuously within a soil profile. It is now well recognized that, even without changes in soil moisture, downward movement of soil moisture is operative for much longer periods of time than could be inferred by the concept of field capacity, and that losses below the root zone should be accounted for in field water balances (Rose and Stern, 1965).

However, drainage of irrigation water below the root zone should not necessarily always be considered as a mere loss but also as a desirable process in alleviating the hazards of salinity, another preponderant consideration in arid and semi-arid lands. On the other hand controlling drainage is important in regulating the movement of soluble nutrients such as nitrates for a more efficient utilization of fertilizer nitrogen and for avoiding nitrate pollution of underground potable water supplies. Thus a precise determination of the drainage component in the field is necessary both for a more accurate estimate of the water balance of a vegetated field and for devising ways and means to regulate the downward movement of irrigation water.

The theoretical aspects of soil water movement have been extensively studied in recent years and the physical principles and the nature of such processes have been reviewed and placed in a logical perspective by Rose (1966) and more recently by Hillel (1971).

The application of theory to field situations awaited the development of tested methods for the determination of soil hydraulic properties in situ. Rose et al (1965) have described one such field method for determining the hydraulic conductivity at any water content by monitoring the internal drainage of a profile in its transient-state. This method was subsequently applied by Rose and Stern (1967) to determine the rate of water uptake by cotton from different depths and at different growth stages. Similarly van Bavel et al (1968 and 1968) have applied a similar approach for the field measurement of unsaturated hydraulic conductivity of fallow, fallow-covered and vegetated plots in an effort to assess the water balance within the root zone of a sorghum crop.

Watson (1966) applied the instantaneous profile method for the determination of the unsaturated hydraulic conductivity of a coarse sand column and concluded that Darcy's Law is applicable to unsaturated flow in similar materials. To determine the hydraulic properties of a draining soil profile Hillel, et al (1972) adapted and tested Watson's method for field situations.

The work described in this paper is an attempt to apply the simplified instantaneous profile method to the same plot in a fallow (covered) - lucerne rotation for the determination of the hydraulic properties, allowing the assessment of the drainage component for which the actual evapotranspiration of lucerne, as determined from soil moisture changes was adjusted.

4.2 EXPERIMENTAL PROCEDURES

4.2.1 Field site characterization

The experimental site selected was within the Institute's farm at Athalassa, near Nicosia, in the central plain where most of the agriculturally exploitable land lies.

The climate of the site bears the accentuated arid character of the central plain in particular and of Cyprus and the eastern Mediterranean basin in general. In dry years the central region receives less than 200 mm of total annual rainfall between November and April and during the dry months of the year the large moisture deficit has to be supplemented by irrigation. The mean monthly maximum temperatures reach as high as 35°C in July-August accompanied by low relative humidity. Evaporation during the summer months is thus particularly favoured reaching 10 mm/day. Fig. 1 typifies this high evaporative demand of the surrounds of the experimental site.

Fig. 1 PAN EVAPORATION (USWB 'A'), CALCULATED ET (Penman) AND MONTHLY RAINFALL (R), ATHLASSA

The soils of the experimental site are medium textured calcareous alluviums classified as calcaric lithosols according to the system adopted by the European Commission on Agriculture of the United Nations. The textural characteristics of the profiles in the experimental plots, referred to hereafter as plot 'A' and plot 'B', are presented in Table 1. It will be noted that texturally neither profile is uniform throughout and that the defined soil layers range from clay loams to sandy loams. In addition, the layer sequence in profiles 'A' and 'B' is not closely comparable although the two plots are only 25 m apart. This is partly the result of land levelling in 1962, but in general most soils in Cyprus would show such variabilities within short distances in the same field.

4.2.2 Plot layout crop rotation and instrumentation

At the commencement of the study in August 1971 plot 'A', located within an established field of lucerne sown in April 1970, covered a 6 x 6 m area. Soon after harvest of lucerne, in the centre of the plot a 270 cm long aluminium access tube (2 inch O.D.) was installed vertically in a hole drilled with a hydraulically operated coring probe of slightly smaller diameter. Mercury tensiometers were installed, with the aid of a screw auger, 50 cm away from the access tube, at 30 cm intervals up to a depth of 240 cm. After each harvest of lucerne the plot and the surrounding buffer zone were irrigated once only to ensure initially a thorough wetting of the profile down to 240 cm. Frequent concurrent moisture and tensiometric records were taken throughout each cycle and until July 1972.

The lucerne was then carefully cut, leaving intact the major portion of the roots, generously irrigated overnight and subsequently covered with a plastic sheet on top of which a mulch of straw followed by a layer of loose soil was placed. The access tube and the battery of tensiometers were left intact. The internal drainage of the fallow (covered) profile was monitored by making frequent moisture measurements and taking tensiometric records over a period of 100 days.

On the other hand plot 'B', 25 m away from 'A' was located in an adjacent fallow field. Plot 'B' was ridged as a 4 x 4 m area surrounded by an outer ridge forming a 2 m-wide buffer zone. Water was ponded on the surface on successive days until steady-state infiltration conditions, as indicated by the tensiometers, were attained.

Table 1 MECHANICAL ANALYSIS, CALCIUM CARBONATE CONTENT AND TEXTURAL GLASS OF SOIL PROFILES 'A' AND 'B' ATHALASSA

	Profile depth (cm)	PH (1:2.5)	Clay %	Silt %	Sand %		CaCO3 %	Textural class
					Pine	Coarse
Athalassa Plot 'A'	0-30	8.6	37.9	25.0	32.2	5.5	20.6	CL
	30-60	8.6	18.9	13.0	56.7	10.3	19.3	SL
	60-90	8.0	22.3	13.8	52.3	8.0	21.1	SCL
	90-120	8.3	32.2	22.9	41.2	3.5	24.3	SCL
	120-150	8.3	23.8	16.2	51.7	7.1	19.5	SCL
	150-180	8.6	23.7	17.7	50.5	6.4	17.7	SCL
	180-210	8.6	39.5	33.5	25.2	2.5	21.7	CL
Athalassa Plot 'B'	0-20	8.4	33.4	32.3	30.5	3.9	19.8	SCL
	20-40	8.2	18.9	17.9	57.3	6.0	16.4	SL
	40-90	8.1	12.0	13.8	60.4	11.8	19.7	SL
	90-120	8.2	26.6	28.3	41.0	4.1	23.0	L
	120-140	8.4	33.3	21.9	40.1	4.9	20.4	SCL
	140-150	8.6	24.0	21.2	50.5	4.3	20.4	SCL
	150-190	8.8	15.7	16.4	56.5	11.4	18.3	SL
	190-210	8.7	21.0	16.4	56.9	5.9	19.2	SCL
	210-250	8.7	35.4	32.7	30.5	1.5	19.4	CL
	250-270	8.5	32.4	31.5	28.7	2.5	18.7	CL
	270-290	8.8	37.9	36.9	25.0	0.4	20.0	CL

CL = Clay loam
SL = Sandy loam
SCL = Sandy clay loam
L = Loam

In April 1972 fallow plot 'B' and the surrounding buffer zone were uncovered, hand cultivated, fertilized with superphosphate and sown to lucerne. The neutron access tube and the battery of tensiometers were left intact. Once the lucerne was established, regular records were taken during each irrigation cycle from July 1972 to July 1973.

In both fallow (covered) plots an additional tensiometer was installed at 5 cm depth. This was not possible in the cropped plots because of the fast drying of the surface top layer. In both cropped plots lucerne was harvested at about 10 percent flowering stage, and the fresh and dry matter of fodder production was recorded and analysed for N, P and K.

4.2.3 Measurement of soil water

With the exception of the top 15 cm, soil water measurements were carried out in situ with the neutron gauge at depth intervals at 15 cm down to 240 cm. The neutron gauge was calibrated against gravimetric sampling and, although in absolute values the instrument indicated higher soil water contents, both the displacement of the calibration curve and of the moisture profiles were parallel over the ranges of soil moisture encountered. Thus the soil moisture changes as determined by the neutron gauge have been accepted as valid.

4.2.4 Measurement of hydraulic head

The hydraulic head was taken as soil suction, measured by tensiometers, plus depth with respect to the soil surface.

4.2.5 Processing of data

The step-by-step handling of the field records of soil moisture changes and suctions, obtained from the fallow (covered) plots, and their subsequent processing followed exactly the same procedural sequence as described by Hillel et al (1972).

Typical measurements in fallow plot 'A' showing the changes of volumetric moisture content and matric suctions with time are depicted in Figs. 2 and 3 respectively. From the moisture-time curves the soil moisture flux through each depth increment down to a specified depth of 195 cm was calculated.

The hydraulic conductivity, K, at each depth and for different soil water contents was calculated by dividing fluxes by the corresponding hydraulic gradients. A plot on semi-log paper, as in Fig. 4, shows the relationship of K to the volumetric moisture content in the various soil layers.

The drainage component in the cropped plots occurring below the depth of 195 cm was computed from the hydraulic conductivities (found from Fig. 4 by reference to the particular moisture contents prevailing in this depth at different days) multiplied by the corresponding hydraulic gradients in the 180-210 cm layer. A typical calculation of the drainage component is given in Table 2.

The changes in soil moisture content occurring in each crop cycle were adjusted for drainage and the rate of actual evapotranspiration (ETa) was calculated and compared to pan evaporation (USWB Class A) as shown in Tables 3 and 4.

4.3 RESULTS AND DISCUSSION

From the mechanical analysis of the various soil layers in both plots it becomes evident that neither profile is texturally uniform throughout the depths considered. Furthermore, the hydrologic characteristics of the constituent layers as depicted in Fig. 2 (changes in soil moisture with time), Fig. 3 (changes in matric suction with time) and particularly by the plots of the relationships of hydraulic conductivities with soil water contents, Fig. 4, show wide variability from layer to layer to the extent that not a single curve but a family of curves will suffice to characterize the profile as a whole. This is true for each profile separately and in comparison to each other.

Fig. 2 CHANGES IN SOIL WATER CONTENT WITH TIME

Fig. 3 CHANGES IN MATRIC SUCTION WITH TIME

Fig. 4 HYDRAULIC CONDUCTIVITIES IN DIFFERENT LAYERS

Table 2 CALCULATION OF THE DRAINAGE COMPONENT, U, DURING AN IRRIGATION CYCLE OF LUCERNE PLOT 'A', ATHALASSA

Period (days)	Q (%)	Hydraulic head, H, at		K (cm/day)
		z=180 cm	z= 210 cm			(cm)
1	45.2	227	306	0.255	2.63	0.67
1	45.7	228	306	0.360	2.60	0.94
1	45.7	228	301	0.360	2.43	0.87
2	44.9	231	293	0.200	2.07	0.83
1	44.7	236	296	0.175	2.00	0.35
1	44.6	238	303	0.160	2.17	0.35
1	44.0	242	309	0.105	2.23	0.23
1	44.0	246	316	0.105	2.33	0.24
1	43.6	250	321	0.085	2.37	0.20
2	43.1	255	325	0.065	2.33	0.30
1	43.1	260	332	0.065	2.40	0.16
1	42.7	265	342	0.053	2.57	0.14
1	42.3	271	351	0.044	2.67	0.12
1	42.2	277	357	0.042	2.67	0.12
1	41.9	282	371	0.037	2.97	0.11
17						5.63

Table 3 CALCULATION OF ACTUAL EVAPOTRANSPIRATION (ETa) ADJUSTED FOR DRAINAGE AND RATE OF DRY MATTER PRODUCTION IN DIFFERENT CROP IRRIGATION CYCLES OF LUCERNE PLOT 'A' (JULY 1971-JULY 1972)

Cycle (days)	dW (mm)	Rainfall (mm)	U (mm)	ETa (mm/day)	E pan (mm/day)	Eta/E pan	Dry matter (kg/day/ha)
17	196	41	-56	10.7	7.4	1.45	86.3
19	193	20	31	9.6	6.9	1.39	89.3
26	212	NIL	27	7.1	6.1	1.16	60.0
37	170	NIL	70	2.7	3.6	0.75	49.5
48	114	91	152	1.1	1.4	0.79	27.0
51	125	68	110	1.6	1.9	0.84	39.8
30	113	48	89	2.4	3.1	0.77	69.0
22	133	12	65	3.7	4.5	0.82	90.0
28	198	17	62	5.5	6.0	0.92	96.0
17	116	38	46	6.3	6.7	0.94	130.5
17	178	NIL	21	9.2	9.1	1.01	176.3
17	200	NIL	33	9.8	8.9	1.10	94.5

dW = change of soil water content within each crop irrigation cycle
U = accumulated drainage in each cycle
E pan = evaporation from open pan USWB Glass 'A'

A typical calculation by this procedure is given in Table 2. It will be observed that the average water content at 195 cm increases slightly 2 or 3 days after irrigation while the hydraulic gradient of the layer 180-210 cm shows a corresponding decrease. This situation is also reflected in the drainage below 195 cm which increased in 2-3 days after irrigation and thereafter diminished to reach a value of 1.1 mm/day on the 17th day.

The distribution and movement of water within the profile after application is demonstrated in Fig. 6 showing that major changes in the cropped plot occur down to a depth of 120 cm while at the depth of 195 cm only comparable minor changes in water content are observed when the plot is cropped or fallow (Fig. 5). In spite of this the hydraulic gradient in the 180-210 cm zone always favoured downward movement of soil water from the depth 195 cm. It is thus concluded that irrespective of any root activity in this zone a net downward movement takes place. This pattern is operative in each and every crop irrigation cycle as shown in Fig. 7 where successive changes in hydraulic head in the different layers and in a number of crop irrigation cycles are plotted against time. In this case too, irrespective of the rate of surface evaporation, the hydraulic gradient between 180 cm and 210 cm always indicates downward movement.

Thus the rate of actual evapotranspiration (ETa) was calculated from the relation proposed by Hillel et al (1972)

where is the change of soil water content within each crop irrigation cycle from the surface down to 195 cm and

is the net accumulated drainage from the same depth, namely the drainage component. The computation of actual evapotranspiration along these lines is shown in Table 3 for lucerne plot 'A' and in Table 4 for lucerne plot ‘B'.

Similar comparisons for lucerne plot 'B' show that the drainage was in general lower than in profile 'A' but the actual evapotranspiration occuring in seasonably corresponding crop cycles were comparable.

Fig. 5 SUCCESSIVE SOIL MOISTURE (a) HYDRAULIC HEAD (b) AMD MATRIC SUCTION (c) IN FALLOW (COVERED) PLOT 'A' ATHALASSA

Fig. 6 SUCCESSIVE SOIL MOISTURE (a) AMD MATRIC SUCTION (b) PROFILES OF PLOT 'A' FOLLOWING IRRIGATION OF LUCERNE 16 JUNE - 3 JULY 1972

Fig. 7 HYDRAULIC HEAD vs. TIME AT FIVE DEPTHS AND MOISTURE vs. TIME AT 180 cm IN LUCERNE PLOT 'A' DURING SIX IRRIGATION CYCLES, ATHALASSA 1973

Table 4 CALCULATION OF ACTUAL EVAPOTRANSPIRATION (ETa) ADJUSTED FOR DRAINAGE AND RATE OF DRY MATTER PRODUCTION IN DIFFERENT CROP IRRIGATION CYCLES OF LUCERNE PLOT 'B' (JULY 1972-JULY 1973)

Cycle (days)	dW (mm)	Rainfall (mm)	U (mm)	ETa (mm/day)	E pan (mm/day)	Eta/E pan	Dry matter (kg/day/ha)
22	239+	NIL	18	10.1	8.8	1.15	83.3
20	224+	5	46	9.1	8.5	1.07	111.0
27	230+	NIL	48	6.8	7.5	0.91	69.5
26	150	NIL	34	4.5	6.3	0.71	51.0
41	141	3	25	2.9	3.2	0.91	42.0
62	77	16	23	1.2	1.8	0.67	27.8
48	91	38	43	1.8	2.8	0.64	63.8
30	191	8	58	4.7	4.8	0.97	114.0
30	171	1	27	4.8	7.2	0.67	130.5
18	115	5	26	5.2	7.5	0.69	192.0
20	164	14	28	7.6	9.4	0.81	169.5
15	179	NIL	17	10.8	10.4	1.04	207.8

+ Received supplementary irrigation of 100 mm
dW = change of soil water content within each crop irrigation cycle
U = accumulated drainage in each cycle
E pan = evaporation from open pan USWB Class 'A'

In general, in both plots the ratio of ETa/Epan approaches or even exceeds unity during the hot summer months and ranges from 0.7 to 0.8 in the winter and cooler periods. A plot of the relationship between ETa and Epan for both plots is shown in Fig. 8. It will be noted that although there is a linearity up to a daily pan evaporation of 7 mm, at values approaching 10 mm per day there seems to be a proportionally higher increase in the rate of actual evapotranspiration.

In this study the field water balance of two lucerne plots was computed from the depletion of soil moisture within each irrigation cycle. This change in soil water content was adjusted for net drainage losses below the root zone. Although the uptake of water from an assumed depth of 195 cm cannot be precluded, yet the net result is that drainage occurs below this depth, albeit at a diminishing rate as depletion of soil moisture increases from regrowth to harvest of lucerne in each crop cycle.

It is realized that in absolute values the calculated drainage component may fluctuate because of its dependence on the hydraulic conductivity which in itself may not be precisely defined in a layered soil. However, this procedure provides an estimate of the actual evapotranspiration of a vegetated field.

ACKNOWLEDGEMENTS

The authors are grateful to the International Atomic Energy Agency for financial and expert assistance.

They also wish to acknowledge the assistance of Messrs. Ph. Kokkinos,

Fig. 8 RELATIONSHIP BETWEEN PAN EVAPORATION (Epan) AMD ACTUAL EVAPOTRANSPIRATION (ETa) FROM TWO LUCERNE PLOTS, ATHALASSA

A. Constantinou and Chr. Xylaris for neutron measurements and other records in the field.

REFERENCES

Fried, M. and Barrada Y. 1967 The need of arid and semi-arid regions for water-use efficiency studies. Ch.I. Soil moisture and irrigation studies (Panel Proceedings Series) IAEA, Vienna.

Gardner, W. R. 1966 Soils water movement and root absorption. Ch.7, Plant environment and efficient water use. Eds. Pierre, W. H., Kirkham, D., Pesek, J and Shaw, R. Amer. Soc. Agron. and Soil Sci. Soc. Amer. Madison.

Gardner, W. R. 1967 Present knowledge of the interrelationships between soil moisture, irrigation, drainage and water-use efficiency. Ch. IV. Soil moisture and irrigation studies (Panel Proceedings Series) IAEA, Vienna.

Hillel, D. 1971 Soil and water: physical principles and processes. Academic Press, New York.

Hillel, D. 1972 et al Procedure and test of an internal drainage method for measuring soil hydraulic characteristics in situ. Soil Sci. 114: 395

Rose, C. W. et al. 1965 Determination of hydraulic conductivity as a function of depth and water content for soil in situ. Aus. J. Soil Res. 3: 1.

Rose, C. W. and Stern, W. R, 1965 The drainage component of the water balance equation. Aus. J. Soil Res. 3:95.

Rose, C. W. 1966 Agricultural physics. Pergamon Press, London.

Rose, C. W. and Stern W. R. 1967 Determination of withdrawal of water from soil by crop roots as a function of depth and time. Aus. J. Soil Res. 5:11.

Stylianou, Y. and Krentos, V. D. 1972 Irrigation requirements of Valencia oranges as affected by the frequency of water application. (Paper IAEA-SM-176-29). FAO/IAEA Symposium on isotopes and radiation techniques in studies of soil physics, irrigation and drainage in relation to crop production. IAEA, Vienna.

van Bavel, C. H. M., Stirk, G. B. and Brust, K. J. 1968 Hydraulic properties of a clay loam soil and the field measurement of water uptake by roots: I. Interpretation of water content and pressure profiles. Soil Sci. Soc. Am. Proc. 32:310.

van Bavel, C. H. M., Brust, K. J. and Stirk, G. B. 1968 Hydraulic properties of a clay loam soil and the field measurement of water uptake by roots: II. The water balance of the root zone. Soil. Sci. Soc. Am. Proc. 32:317

Watson, K. K. 1966 An instantaneous profile method for determining the hydraulic conductivity of unsaturated porous materials. Water Resources Res. 2:709

2. Some physical properties of sandy soils and sand dunes in Iraq

by
Jamal S. Dougrameji ^1/
Soil-Water Division
Arab Centre for the Studies of Arid Zones and Dry Lands

^1/Formally Assistant Professor and Director, Institute for Applied Research on Natural Resources, Abu Ghraib, Iraq.

In Iraq, sandy soils and sand dunes are located around Baiji in the north, in an area north-east of Hilla - Diwaniyah and in a sand belt, more or less parallel to the Euphrates situated in a NW-SE direction south of a line Najaf Zubair with a width ranging from 5-25 km. The general movement of dunes is in a south-easterly direction as a result of prevailing north-westerly winds (Map 1).

Among the many factors responsible for soil drift and the consequent formation of sand dunes are drought, over-exploitation of natural vegetation, the unprotected dredged material along canals and improper operations. These factors led to serious socio-economic problems, the engulfing of agricultural fields and grazing lands, irrigation systems and lines of communication by blown sand from adjoining areas.

Before attempting large scale phyto-reclamation of sandy soils and shifting dunes, it is well worth investigating their plant growing conditions with special emphasis on soil moisture characteristics in relation to particle size distribution, rainfall distribution and intensity in relation to soil moisture distribution at different depths and after each rain. These relationships plus the information on natural vegetation and climate will provide a better understanding of the ecosystem from the point of view of its reclamability through vegetation as well as a sound criterion for the choice of plant species to be used during the process of reclamation.

This report deals with the results of investigations carried out by the author on the sandy soils and sand dunes of Iraq.

2.2 MATERIAL AND METHODS

Soil moisture characteristics of graded silica sands were determined in triplicate samples on suction tables. The measurements were made at the same intervals from 0-60 cm suctions (Dougrameji, 1965).

Soil moisture characteristics, bulk density and percent porosity were obtained in triplicate samples of Zubair soils using suction tables in the intervals of 0-100 cm water suction, and pressure plate and pressure membrane extractor for pressures ranging from 1/3 to 15 atmosphere (Dougrameji, 1970)

Soil moisture characteristics and particle size distribution were determined on soil samples from dunes in Baiji, Mussayeb, Najaf, Muggaishi, Massiriyah, Amara and Zubair. In addition, two samples from the sediment of the Mussayeb and Hamir canals in the Greater Mussayeb project were also studied in order to determine whether these sediments constitute the source of the material forming dunes in the project (Dougrameji and Kaul 1971).

IRAQ - LOCATION OF SAND DUNES & WIND DIRECTION - Map 1

2.3 RESULT AND DISCUSSION

The data in Table 1 show an increase in soil moisture content for the uniform sand separates as the particle size of the separates decreases. The suction required to drain the pores in the sand separate increases with decreasing particle size.

The percent soil moisture for layered samples in Tables 2 and 3 indicates the presence of two breaks in soil moisture suction curves. The breaks become more pronounced the greater the difference in particle size of the two layers. But when the fine sand underlay the coarse sand, the coarse sand layer drained out through the fine layer at almost its normal suction, then the bottom fine layer drained when the suction was raised. On the other hand, when the coarse sand underlay the fine sand there was no significant amount of water removed from the sample until the suction necessary to drain the fine layer was reached. However, after approaching this critical point, a small increase in suction caused removal of a large amount of water from the sample due to draining of both layers at once.

Table 1 PERCENT MOISTURE BY HEIGHT AT VARIOUS SUCTIONS IN CORES OF UNIFORM SAND SEPARATE

Average size particle mm dia	Moisture suction -cm water							Moisture distribution ratio at 60 cm suction
	0	10	20	30	40	50	60
	Percent moisture by weight
2.20	21.1	2.3	1.4	0.9	0.7	0.6	0.5	1.0
1.34	22.1	2.5	1.2	0.8	0.6	0.5	0.7	1.0
0.63	29.7	16.6	3.2	2.2	1.6	1.0	0.8	1.0
0.23	34.7	34.0	33.3	21.9	7.2	4.2	3.3	1.0

Table 2 PERCENT MOISTURE BY WEIGHT AT VARIOUS SUCTIONS IN CORES CONSISTING OF TWO LAYERS (COARSE SAND OVER FINE SAND SEPARATES)

Average particle size of top and bottom layers mm dia	Moisture suction -cm water							Moisture distribution ratio at 60 cm suction
	0	10	20	30	40	50	60
	Percent moisture by weight
2.20/0.63	18.7	13.5	2.3	1.5	1.1	0.8	0.7	0.55
2.20/0.23	22.3	17.7	17.0	15.3	4.4	2.6	2.1	0.10
1.34/0.63	23.9	13.9	2.4	1.6	1.2	0.9	0.7	0.70
1.34/0.23	27.5	17.9	17.1	15.6	4.5	2.6	2.1	0.10
0.63/0.23	32.4	22.2	17.9	15.1	4.5	2.7	2.1	0.10

Table 3 PERCENT MOISTURE BY WEIGHT AT VARIOUS SUCTIONS IN CORES CONSISTING OF TWO LAYERS (FINE OVER COARSE SAND SEPARATES)

Average particle size of top and bottom layers mm dia	Moisture suction - cm water							Moisture distribution ratio at 60 cm suction
	0	10	20	30	40	50	60
	Percent moisture by weight
1.34/2.20	24.6	2.5	1.6	1.1	0.9	0.8	0.6	0.0
0.63/2.20	26.4	13.5	4.4	9.7	8.8	8.2	7.6	15.2
0.23/2.20	27.0	26.2	24.7	15.9	14.9	14.5	14.3	28.6
0.63/1.34	26.9	26.3	14.5	12.7	11.7	10.8	10.3	26.3
0.23/1.34	28.8	28.2	27.7	17.5	16.9	16.3	16.1	87.1
0.23/0.63	31.6	30.5	19.0	17.8	16.8	16.0	15.2	23.6

The moisture distribution in the drained sand layers indicated the presence of a large amount of water in the fine layer overlying the coarse layer. The moisture content was 75% higher when the fine sand of 0.23 mm average particle size was underlain by a very coarse sand of 1.34 mm in diameter.

Zubair soils consist of gravelly sands developed by aeolian sorting and deposition. The soil profiles studied were characterized by light greyish brown loamy sands surface soil underlain by light brownish yellow sands. In the dominantly coarse-textured soils mechanical composition of the various profiles ranged from 80-95% sand, 1.5-12.8% silt and 2.7-10.5% clay. The range of available moisture of different layers is given in Table 4. The amount of soil moisture retained was mostly available at suctions less than 6.0 atm.

Table 4 SOIL MOISTURE CHARACTERISTICS OF SOIL PROFILES OF ZUBAIR (IRAQ)

Soil depth cm	Soil Texture	Range of moisture % by weight			Range of available moisture % by weight
		1/3 atm	6 atm	15 atm	1/3-6 atm	1/3-15 atm
0- 30	L.S.	8.9-10.9	2.7-3.6	2.3-2.9	6.2-7.3	6.6-8.0
30- 60	L.S. to S	6.0-8.3	2.3-2.4	1.8-1.9	3.7-5.9	4.2-6.4
60- 90	S	4.7- 6.7	1.9-2.2	1.5-1.8	2.8-4.5	3.2-4.9
90-120	S	4.2-4.8	1.8-1.9	1.4-1.6	2.4-2.9	2.8-3.2

The texture of non-clay soil separates ranged from coarse silt to very coarse sand. The bulk density ranged from 1.4 to 1.64 g/cc and decreased with decreasing particle size. The particle density was almost constant and averaged 2.69 g/cc.

Table 5 PERCENT MOISTURE BY WEIGHT AT VARIOUS SOIL MOISTURE SUCTIONS IN CORES OF DIFFERENT NATURAL SAND SEPARATES (ZUBAIR, IRAQ)

Particle Size		Soil moisture suction - cm water or atm
mm.	dia	0	10	20	30	60	1/3 atm	1 atm	3 atm	6 atm	15 atm
		Percent moisture by weight
2.00 - 1.00		26.53	6.14	4.58	4.10	3.06	1.68	1.28	0.74	0.38	0.29
1.00 - 0.50		27.69	10.29	6.29	5.14	3.56	1.83	1.60	1.00	0.61	0.51
0.50 - 0.25		29.22	24.80	21.24	9.92	5.05	2.49	1.99	1.48	1.24	0.68
0.25 - 0.105		30.44	28.25	26.94	25.25	16.43	4.95	3.34	2.20	1.75	1.22
0.105 - 0.053		31.95	30.35	29.37	28.93	28.10	19.45	11.85	4.58	2.90	2.45
0.053 - 0.020		34.75	33.75	33.38	32.87	32.15	27.38	18.75	8.30	3.84	2.98

Table 5 gives the moisture content expressed in percent by weight for each sand separate over the entire soil suction range that permits plants to grow. The data illustrate that the percentage of moisture was increased and the suction required to drain the pores in the sand separates increased with decreasing particle diameter.

The data show that the amount of soil moisture retained at equal suctions differs from one sand separate to another. The separate with a particle diameter larger than 0.5 mm released most of its water at a soil water suction range from 0-10 cm, but the separate 1.0-0.5 mm retained 4% of its available moisture at a range of 10-20 cm soil water suction. The separate 0.25-0.105 needed higher suction to release its water (30-100 cm water suction).

In the separate with a particle diameter smaller than 0.105 mm no significant amount of water was released below 100 cm soil water suction, and most of the moisture was retained in the range between 100 and 300 cm soil water suction.

The data in Table 6 show that the calculated available moisture between 1/3-6 atm and 1/3-15 atm are almost the same and to a lesser extent the data in Table 7 These results suggest that further studies be made on the upper limits of available water in sandy soils.

Table 6 PERCENT AVAILABLE MOISTURE IN UNIFORM SAND SEPARATES OF ZUBAIR WHEN COMPARED AT TWO RANGES OF SOIL WATER-SUCTION

Particle	Percent available moisture between
size - mm	1/3 - 6 atm	1/3 - 15 atm
2.0 - 1.0	1.30	1.39
1.0 - 0.5	1.22	1.32
0.5 - 0.25	1.25	1.81
0.25 - 0.105	3.20	3.73
0.105 - 0.053	16.55	17.00
0.053 - 0.002	23.54	24.40

In the studied sand dunes a wide variation in the distribution of particle size was found. The soil from all the dunes is sandy except that of Amara 2, Muggaishi and Mussayeb 1 and 2 which are formed of sandy clay loam, loamy sand clay, loam and sandy loam respectively. Comparing the particle size distribution in the Mussayeb dunes and the dredged sediments from the two irrigation canals in the Mussayeb project revealed that more than 50% of the particles constituting the dunes are similar. The dunes in Zubair and Najaf compared to the dunes in other localities showed a higher percentage (8.0 and 8.7 respectively) of coarse sand. Except the dunes of Amara 2 and Mussayeb 1 and 2, the dunes in all other localities are largely composed of very fine sand to medium sand (80% of the particle size being 0.1-0.5 mm).

Table 7 SOIL MOISTURE CHARACTERISTICS OF SOILS FROM DUNES AND MUSSAYEB CANAL SEDIMENTS IN IRAQ

Location	% moisture by wt - atm			Bulk density	% available moisture by
					volume	atm
	1/3 atm	6 atm	15 atm	g/cc	1/3-6 atm	1 /3-15 atm
Baiji-1	7.9	2.6	1.9	1.6	8.5	9.6
Baiji-2	6.1	2.2	1.6	1.6	6.24	7.2
Mussayeb-1	18.5	12.0	9.5	1.3	8.45	11.7
Mussayeb-2	15.0	8.8	6.8	1.27	7.87	10.4
Muggaishi	12.8	4.4	3.6	1.42	12.93	13.1
Nassiryah-1	7.3	2.2	1.5	1.63	8.3	9.5
Nassiryah-2	5.4	1.6	1.0	1.69	6.4	7.4
Amara-1	6.6	2.5	1.8	1.48	6.07	7.1
Amara-2	15.4	5.4	3.7	1.33	13.3	15.6
Najaf	7.7	2.0	1.1	1.71	9.75	11.3
Zubair-1	5.9	1.6	1.0	1.69	7.27	8.3
Zubair-2	13.4	6.7	5.2	1.30	8.71	10.6
Mussayeb	12.5	3.6	2.3	1.44	12.8	14.7
Canal Sediments	13.1	4.2	2.8	1.44	12.8	14.8

The maximum available moisture by volume in the different dunes (Table 7) varied from 7.2 in Baiji to 15.6 in Amara 2. As expected, the dunes composed of sandy clay loam and clay loam with a high percent of clay exhibited a high water holding capacity and higher available moisture compared to those composed of sand.

The results of these studies explain the importance of soil texture in determining soil moisture holding capacity, available moisture to plants and irrigation requirements; for example, the early concept of field capacity excluded from generalization those soils which are known to restrict downward flow of water. The texture changes in the profile were usually disregarded in describing field capacity; furthermore it was found that the flow of water will continue from the surface layer downward beyond the defined field capacity. Also growing plants play an important role in extracting the available water (Dwane and Loomis, 1967). For this reason, the field capacity should be considered as a soil moisture profile and the growing plant function rather than a property of soil in the root zone or the plough layer. Therefore, it appears that in sandy soils and non-clay sand separates, the available moisture has different ranges of pressure for which the moisture is available for plant growth and should be considered in irrigation practices. The available moisture of these soils when compared with the mean annual rainfall of 150 mm in the region (during the winter season) indicates moisture as the most important limiting factor, especially during the dry summer months, for plant establishment and growth,

4. SUMMARY AND CONCLUSIONS

Soil moisture characteristics of graded silica sand, Zubair sandy soils and sand dunes from different locations in Iraq were studied.

The results of these studies indicate that:

a. although the moisture suction required to drain a soil depends on its particle size, in stratified soils the size of particle in a coarse stratum can govern the movement and retention of water;

b. in the sandy soils and the soils studied, most of the available moisture is retained at suctions less than 6 atm and it seems that this value is the maximum value to use under favourable field conditions for the sandy soils in estimating available moisture.

c. study of rainfall characteristics and the soil moisture regime through the soil profile should always be thoroughly considered in sand dune reclamation.

Dougrameji, J. S. 1965 Soil - water relationships in stratified sands. Ph.D. thesis. Michigan State University.

1970 The relation of particle size to available moisture in soils of Zubair. Annals of Arid Zone Vol. 9 No. 3

1971 Report on the possibilities of reclamation of sand dunes and proposed future research. Institute for Research on Natural Resources, Abu Ghraib, Iraq.

Dwane, J. S. and Loomis, W. E. 1967 Plant and soil factors in permanent wilting percentages and field capacity storage. Soil Sci. 104:3

Learner, R. W, and Shaw, B. A. 1941 Simple apparatus for measuring non-capillary porosity on an extensive scale. J. Agr. Soc. Agron. 33: 1003-1008

Miller, D. E. and Bunger, W. C. 1963 Moisture retention by soils with coarse layers in the profile. Soil Sci. Soc. Amer. Proc. 27

Rechard, L. A. 1949 Methods of measuring soil moisture tension. Soil Sci. 68:95-112

Stakman, W. P. 1966 The relation between particle size, pore size and hydraulic conductivity of sand separates. Symposium on water in the unsaturated zone, Wageningen.

Tippetts-Abbett-McCarthy-Stratton. 1957 Report on the Zubair irrigation project, Basrah Vicinity, Iraq.

3. The sandy soils of the Kingdom of Saudi Arabia

by
P. Loizides
FAO Adviser, Central Agricultural Laboratories
and Water Use in the Kingdom of Saudi Arabia

The Kingdom of Saudi Arabia is one of the hottest and driest regions of the earth. In the mountain and coastal areas of the south-western part of the country rainfall rises to 300-400 mm. This allows dry farming to be practised. Over the greatest part of the country, however, average rainfall amounts to only 20-150 mm and agriculture is only possible under irrigation.

In the inland regions summer temperatures in the day-time regularly exceed 40°C, while in winter freezing temperatures frequently occur. Relative humidity in the day-time during summer drops to 10-20%. In the coastal regions relative humidity is much higher and temperature differences between summer and winter less pronounced. Strong dry winds frequently blow especially in summer.

Irrigated agriculture has been practised for thousand of years from shallow wells tapping the shallow unconfined aquifers of the alluvial deposits along the numerous wadi beds or from springs, notably those of the el Hassa oasis, the flow of which amounts to 15 m³/sec. The shallow aquifers are regularly replenished by wadi floods and recharge usually exceeds extraction. Recharge may be increased by flood control structures but the development potential of the small unconfined aquifers is limited.

During the last few decades huge quantities of underground water have been discovered. They are stored in confined aquifers of the Cretaceous to Pleistocene formations dipping off the ancient crystalline rocks of the Arabian shield towards the Arabian Gulf 500 km to the east. The water is fossil having accumulated during an earlier pluvial period and very little replenishment is now taking place. Properly managed it may perhaps allow in area of 100 000 ha to be irrigated over a period of 50-100 years in addition to the 100 000 ha or so now under irrigation. If pumping is done from depths greater than 150-200 m and if desalinization of the more brackish waters is practised much bigger areas could probably be irrigated.

The main crops are date palms, cereals, fodder crops such as alfalfa and sorghum and vegetables. In certain areas deciduous fruit trees, citrus and vines are also grown.

The quality of water varies but in general the salt content of the deep confined waters is 800 to 6 000 ppm rising towards the coast of the Arabian Gulf. The quality of water in the small wadi aquifers is even more variable increasing downstream as well as away from the wadi bed. Little information is available about the boron content of the irrigation waters.

No basic soil survey has been carried out in the Kingdom. Reconnaissance land capability surveys based on the U.S. Bureau of Reclamation system provide incomplete information on the soils of the Kingdom. Agricultural soils belong to the loamy-sand to sandy loam textural classes with the lighter soils predominating. Infiltration rates are high amounting to 5-16 cm per hour while the field capacity determined in the field varies from 7% to over 15% by volume. The soils are always calcareous and often contain gypsum. A horizon of lime accumulation is often present.

The main problem in the management of those soils is efficient irrigation. Evapotranspiration during summer probably exceeds 10 mm per day. In order to avoid high percentage depletion of available water in these soils of low field capacity it is necessary to irrigate every 2-3 days. Taking into account the leaching requirements in order to control salinity, the depth of water per application should not exceed 4-6 cm in the lighter soils and 6-8 in the heavier soils. It is extremely difficult to ensure uniform distribution of such small depths by surface methods of irrigation, especially since land preparation is often unsatisfactory. As a result, much water is wasted. Even in the small irrigation basins often used for irrigation, which in theory should result in high efficiencies, the farmer tends to give much more water than is needed to meet evapotranspiration plus leaching requirements.

Sprinkler and for row crops, trickle irrigation would presumably result in much higher efficiencies but such methods have not yet been tried in the Kingdom.

Certain areas suffer from a high water-table. This problem is especially acute in parts of the important agricultural region of Qassim, 400 km N.W. of Riyadh, the el Hassa Oasis and the flat coastal region along the Arabian Gulf near Dammam. Drainage is necessary for dealing with this problem. In el Hassa a drainage system has been laid down as part of a big project which has also provided for the distribution of water by means of concrete channels. However, field drains have not yet been laid.

At the Qatif Agricultural Research Station near Dammam the drainage system is working satisfactorily. In Qassim the drainage problem remains acute. An interesting attempt at reclaiming a saline soil with a high water-table was made by a farmer in this area. He applied a layer of sand 5-10 cm deep to his land and has since succeeded in obtaining much higher yields of cereals and alfalfa than before. It would seem that this success is due to the fact that only a small part of the depression is under crops so that under irrigation there is a net movement of water away from the irrigation fields, while the added sand provides more favourable conditions for germination and growth than the original saline soil. If the entire depression is to be reclaimed an efficient drainage system is needed.

Occasionally, problems may arise due to impermeable calcareous hardpans. This is perhaps true of parts of el Hassa. At the Qatif Agricultural Research Station a hardpan consisting mainly of gypsum originally caused trouble but after some years of drainage and irrigation it has largely dissolved away and drainage is now, if anything, excessive.

Soil salinity is easily controlled under irrigation in well drained soils of light texture but the return flow to the shallow unconfined aquifer may cause salinization of the irrigation water unless this is compensated by sufficient underflow. This appears to be happening in the large el Harj aquifer, 100 km S.E. of Riyadh which is recharged by a number of wadis. As a result of overpumping the water-table is falling and there is no longer any underflow away from the aquifer. The water is therefore being continuously concentrated by evaporation and its salinity is rising,

Certain soils, notably those of Qatif, contain gypsum to the extent of 30% or more. Germination and growth of many vegetables is very poor in such soils. It has been found that heavy applications of poultry manure result in highly increased yields. Since this material contains no more than 10% of organic matter its effect is probably purely mechanical by preventing the formation of a thin gypsum crust by evaporation between irrigations. Excellent yields of many vegetables are obtained at this station. The result should prove of interest to other Middle East countries where similar gypsum soils occur.

Moving sand dunes cover huge areas. They often constitute a hazard by invading agricultural lands. In el Hassa they have been successfully fixed by planting tamarisc after spreading a thin layer of heavier soil over the sand dunes. Irrigation is given for a few years. The success of this system is probably due to the proximity of a watertable which is eventually tapped by the deep-rooted tamarisc, otherwise it would not survive under a rainfall of 30-50 mm. More recent work in this area indicates that it is not necessary to apply a layer of heavier soil before planting but protection against wind is necessary. Alongside highways asphalt is used for fixing moving sand dunes.

Certain hydrologists believe that considerable recharge of the aquifers takes place in these sand dunes since during the occasional heavy rains a large part of the rainfall percolates deep into the soil, escapes evaporation from the surface and can only continue moving downwards. The FAO hydrologist, Mr. Turgut Tincer, measured the tritium content of soil water at various depths and concluded that recharge in the dunes along the Riyadh-Dammam road may be of the order of 20 mm per annum over an area of 25 000 km².

The fertility of the soils of Saudi Arabia is surprisingly high. Experiments carried out by the FAO experts, Messrs. Bolle-Jones and Boswinkle indicate, as expected, a high response of most crops to nitrogen, but phosphorus fertilizers did not always raise yields while in only one experiment did potassium fertilizers raise the yield of late outs of alfalfa. Fertilizer experiments are now in progress at the Qatif Station but a comprehensive programme covering all important agricultural regions and crops is indicated. In particular, experiments are needed on split applications and on new forms of nitrogen fertilizers with a view to reducing the considerable loss of nitrogen that takes place in sandy soils by leaching. The design of the experiments and the level of dressings should be such as to allow the economic optimum to be estimated.

During summer, nodulation of alfalfa is restricted or absent. It is not yet known whether this is due to the absence of suitable strains of bacteria or to the high soil temperatures. The problem is worth studying in view of the importance of this crop.

Trace element deficiencies are present, in particular zinc deficiency in citrus and probably other fruit trees. On the other hand, iron deficiency is of minor importance, being observed only in sensitive plants such as lemon trees growing in the presence of a high watertable. This is to be expected in view of the good aeration of the roots and the fact that the calcium carbonate content of sandy soils occurs mainly in the coarse fractions. It is a matter of experience that iron deficiency is most common in soils of high water holding capacity and in which a large proportion of the calcium carbonate is present in the silt and clay fractions. In general, in spite of the light texture of the soils, minor element deficiencies appear to be of less importance in Saudi Arabia than in other countries of the Middle East.

Both zinc and iron deficiencies are easy to correct; the first by zinc sulphate sprays and the second by means of soil applications of sequestrene Fe 138 chelate. Although this material is very expensive its use is probably justified economically for fruit trees but not for field crops.

In conclusion, mention should be made of a serious disability militating against efficient land and water use. This is the extreme parcellation of holdings and small size of plots especially in areas of traditional agriculture. A radical removal of this disability would be extremely difficult to achieve but perhaps some form of cooperation between owners of neighbouring plots may be reached so as to ensure a more efficient arrangement of plots for irrigation and a more efficient lay-out of irrigation canals and field drains. This problem is widespread in the Middle East and the experience of other countries in solving it may prove of value under the conditions of the Kingdom of Saudi Arabia.