2. Water and soil requirements

2.1 Water requirements of crops
2.2 Water requirements of trees, rangeland and fodder
2.3 Soil requirements for water harvesting

2.1 Water requirements of crops

2.1.1 Introduction
2.1.2 General estimates
2.1.3 Factors influencing crop water requirements
2.1.4 Calculation of crop water requirements

2.1.1 Introduction

For the design of water harvesting systems, it is necessary to assess the water requirement of the crop intended to be grown.

There have been various methods developed to determine the water requirement for specific plants. An excellent guide to the details of these calculations and different methods is the FAO Irrigation and Drainage Paper 24 "Crop Water Requirements". It should however be noted that formulae which give high accuracy also require a high accuracy of measured input data which in most places where water harvesting is practised will not be available.

2.1.2 General estimates

In the absence of any measured climatic data, it is often adequate to use estimates of water requirements for common crops (Table 2). However, for a better understanding of the various factors and their interrelationship which influences the water demand of a specific plant, the following has been drawn from the FAO Irrigation Water Management Training Manual No. 3.

Table 2 - APPROXIMATE VALUES OF SEASONAL CROP WATER NEEDS

Crop	Crop water need (mm/total growing period)
Beans	300 - 500
Citrus	900 - 1200
Cotton	700 - 1300
Groundnut	500 - 700
Maize	500 - 800
Sorghum/millet	450 - 650
Soybean	450 - 700
Sunflower	600 - 1000

2.1.3 Factors influencing crop water requirements

i. Influence of climate

A certain crop grown in a sunny and hot climate needs more water per day than the same crop grown in a cloudy and cooler climate. There are, however, apart from sunshine and temperature, other climatic factors which influence the crop water need. These factors are humidity and wind speed. When it is dry, the crop water needs are higher than when it is humid. In windy climates, the crops will use more water than in calm climates.

The highest crop water needs are thus found in areas which are hot, dry, windy and sunny. The lowest values are found when it is cool, humid and cloudy with little or no wind.

From the above, it is clear that the crop grown in different climatic zones will have different water needs. For example, a certain maize variety grown in a cool climate will need less water per day than the same maize variety grown in a hotter climate.

Plate 1 - Wilted maize crop

Plate 2 Good maize crop

It is therefore useful to take a certain standard crop or reference crop and determine how much water this crop needs per day in the various climatic regions. As a standard crop (or reference crop) grass has been chosen.

Table 4 indicates the average daily water needs of this reference grass crop. The daily water needs of the grass depend on the climatic zone (rainfall regime) and daily temperatures.

Table 3 - EFFECT OF MAJOR CLIMATIC FACTORS ON CROP WATER NEEDS

Climatic factor	Crop water need
	High	Low
Sunshine	sunny (no clouds)	cloudy (no sun)
Temperature	hot	cool
Humidity	low (dry)	high (humid)
Wind speed	windy	little wind

Table 4 - AVERAGE DAILY WATER NEED OF STANDARD GRASS DURING IRRIGATION SEASON (mm)

Climatic zone	Mean daily temperature
	low (< 15°C)	medium (15-25°C)	high (> 25°C)
Desert/arid	4-6	7-8	9-10
Semi-arid	4-5	6-7	8-9

For the various field crops it is possible to determine how much water they need compared to the standard grass. A number of crops need less water than grass, a number of crops need more water than grass and other crops need more or less the same amount of water as grass. Understanding of this relationship is extremely important for the selection of crops to be grown in a water harvesting scheme (see Chapter 6, Crop Husbandry).

Table 5 - CROP WATER NEEDS IN PEAK PERIOD OF VARIOUS CROPS COMPARED TO THE STANDARD GRASS CROP

-30%	-10%	same as standard grass	+10%	+20%
Citrus Olives	Squash	Crucifers Groundnuts Melons Onions Peppers Grass Clean cultivated nuts & fruit trees	Barley Beans Maize Cotton Lentils Millet Safflower Sorghum Soybeans Sunflower Wheat	Nuts & fruit trees with cover crop

ii. Influence of crop type on crop water needs

The influence of the crop type on the crop water need is important in two ways.

a. The crop type has an influence on the daily water needs of a fully grown crop; i.e. the peak daily water needs of a fully developed maize crop will need more water per day than a fully developed crop of onions.

b. The crop type has an influence on the duration of the total growing season of the crop. There are short duration crops, e.g. peas, with a duration of the total growing season of 90-100 days and longer duration crops, e.g. melons, with a duration of the total growing season of 120-160 days. There are, of course, also perennial crops that are in the field for many years, such as fruit trees.

While, for example, the daily water need of melons may be less than the daily water need of beans, the seasonal water need of melons will be higher than that of beans because the duration of the total growing season of melons is much longer.

Data on the duration of the total growing season of the various crops grown in an area can best be obtained locally. These data may be obtained from, for example, the seed supplier, the Extension Service, the Irrigation Department or Ministry of Agriculture.

Table 6 gives some indicative values or approximate values for the duration of the total growing season for the various field crops. It should, however, be noted that the values are only rough approximations and it is much better to obtain the values locally.

Table 6 - INDICATIVE VALUES OF THE TOTAL GROWING PERIOD

Crop	Total growing period (days)	Crop	Total growing period (days)
Alfalfa	100-365	Melon	120-160
Barley/Oats/ Wheat	120-150	Millet	105-140
Bean, green	75-90	Onion, green	70-95
dry	95-110	dry	150-210
Citrus	240-365	Pepper	120-210
Cotton	180-195	Rice	90-150
Grain/small	150-165	Sorghum	120-130
Lentil	150-170	Soybean	135-150
Maize, sweet	80-110	Squash	95-120
grain	125-180	Sunflower	125-130

From Table 6, it is obvious that there is a large variation of values not only between crops, but also within one crop type. In general, it can be assumed that the growing period for a certain crop is longer when the climate is cool and shorter when the climate is warm.

Crops differ in their response to moisture deficit. This characteristic is commonly termed "drought resistance" (Table 7 summarizes sensitivity to drought). When crop water requirements are not met, crops with a high drought sensitivity suffer greater reductions in yields than crops with a low sensitivity.

Table 7 - GENERAL SENSITIVITY TO DROUGHT

Group One:	(low sensitivity)	Groundnuts
	¯	Safflower
Group Two:	¯	Sorghum
	¯	Cotton
	¯	Sunflower
Group Three:	¯	Beans
Group Four:	(high sensitivity)	Maize

2.1.4 Calculation of crop water requirements

i. Introduction

The calculation of crop water requirements by means of the two methods described in this section is relatively simple. The basic formula for the calculation reads as follows:

ETcrop = kc x Eto

where:

ETcrop = the water requirement of a given crop in mm per unit of time e.g. mm/day, mm/month or mm/season.

kc = the "crop factor"

ETo = the "reference crop evapotranspiration" in mm per unit of time e.g. mm/day, mm/month or mm/season.

ii. ETo - reference crop evapotranspiration

The reference crop evapotranspiration ETo (sometimes called potential evapotranspiration, PET) is defined as the rate of evapotranspiration from a large area covered by green grass which grows actively, completely shades the ground and which is not short of water. The rate of water which evapotranspirates depends on the climate. The highest value of ETo is found in areas which are hot, dry, windy and sunny whereas the lowest values are observed in areas where it is cool, humid and cloudy with little or no wind.

In many cases it will be possible to obtain estimates of ETo for the area of concern (or an area nearby with similar climatic conditions) from the Meteorological Service. However, where this is not possible, the values for ETo have to be calculated. Two easy methods will be explained below:

a. Pan evaporation method

With this method, ETo can be obtained by using evaporation rates which are directly measured with an evaporation pan. This is a shallow pan, containing water which is exposed to the evaporative influence of the climate. The standard pan is the Class A Pan of the US Weather Bureau (Figure 6). It has a diameter of 1.21 m, a depth of 25 cm and is placed 15 cm above the ground.

Figure 6 Class A evaporation pan

An evaporation pan is easy to construct and in most situations the material can be found locally.

The principle of obtaining evaporation rates from the pan is as follows:

- the pan is installed in the field 15 cm above the ground;

- the pan is filled with water 5 cm below the rim;

- the water is allowed to evaporate during a certain period of time (usually 24 hours). For example, each morning at 7.00 hours a measurement is taken. Rainfall, if any, is measured simultaneously;

- after 24 hours, the water depth is measured again;

- the amount of water which has evaporated in a given time unit is equal to the difference between the two measured water depths. This is the pan evaporation rate: Epan (mm/24 hours).

The readings taken from the pan (Epan) however do not give ETo directly, but have to be multiplied by a "Pan Coefficient" (Kpan).

thus: ETo = Epan x Kpan

For the Class A evaporation pan, Kpan varies between 0.35 and 0.85, with an average of 0.70. If the precise pan factor is not known, the average value (0.70) can be used as an approximation. For greater accuracy a detailed table of Kpan figures is given in Irrigation Water Management Training Manual No. 3.

b. The Blaney-Criddle Method

If no measured data on pan evaporation are available, the Blaney-Criddle method can be used to calculate ETo. This method is straightforward and requires only data on mean daily temperatures. However, with this method, only approximations of ETo are obtained which can be inaccurate in extreme conditions.

The Blaney-Criddle formula is: ETo = p(0.46Tmean + 8) where:

ETo = reference crop evapotranspiration (mm/day)
Tmean = mean daily temperature (° C)
p = mean daily percentage of annual daytime hours.

The Blaney-Criddle Method always refers to mean monthly values, both for the temperature and the ETo. If in a local meteorological station the daily minimum and maximum temperatures are measured, the mean daily temperature is calculated as follows:

To determine the value of p. Table 8 is used. To be able to obtain the p value, it is essential to know the approximate latitude of the area: the number of degrees north or south of the Equator.

Table 8 - MEAN DAILY PERCENTAGE (p) OF ANNUAL DAYTIME HOURS FOR DIFFERENT LATITUDES

Latitude:
North	Jan	Feb	Mar	Apr	May	Jun	July	Aug	Sept	Oct	Nov	Dec
South	July	Aug	Sept	Oct	Nov	Dec	Jan	Feb	Mar	Apr	May	June
60°	.15	.20	.26	.32	.38	.41	.40	.34	.28	.22	.17	.13
55	.17	.21	.26	.32	.36	.39	.38	.33	.28	.23	.18	.16
50	.19	.23	.27	.31	.34	.36	.35	.32	.28	.24	.20	.18
45	.20	.23	.27	.30	.34	.35	.34	.32	.28	.24	.21	.20
40	.22	.24	.27	.30	.32	.34	.33	.31	.28	.25	.22	.21
35	.23	.25	.27	.29	.31	.32	.32	.30	.28	.25	.23	.22
30	.24	.25	.27	.29	.31	.32	.31	.30	.28	.26	.24	.23
25	.24	.26	.27	.29	.30	.31	.31	.29	.28	.26	.25	.24
20	.25	.26	.27	.28	.29	.30	.30	.29	.28	.26	.25	.25
15	.26	.26	.27	.28	.29	.29	.29	.28	.28	.27	.26	.25
10	.26	.27	.27	.28	.28	.29	.29	.28	.28	.27	.26	.26
5	.27	.27	.27	.28	.28	.28	.28	.28	.28	.27	.27	.27
0	.27	.27	.27	.27	.27	.27	.27	.27	.27	.27	.27	.27

For example, when p = 0.29 and T mean = 21.5 °C, the ETo is calculated as follows: ETo = 0.29 (0.46 x 21.5 + 8) = 0.29 (9.89 + 8) = 0.29 x 17.89 = 5.2 mm/day.

Table 9 - INDICATIVE VALUES OF Eto (mm/day)

Climatic zone	Mean daily temperature
	15°	15-25°C	25°
Desert/arid	4-6	7-8	9-10
Semi-arid	4-5	6-7	8-9
Sub-humid	3-4	5-6	7-8
Humid	1-2	3-4	5-6

c. Indicative values of ETo

Table 9 contains approximate values fur ETo which may be used in the absence of measured or calculated figures.

iii. Crop Factor - Kc

In order to obtain the crop water requirement ETcrop the reference crop evapotranspiration, ETo, must be multiplied by the crop factor, Kc. The crop factor (or "crop coefficient") varies according to the growth stage of the crop. There are four growth stages to distinguish:

- the initial stage: when the crop uses little water;
- the crop development stage, when the water consumption increases;
- the mid-season stage, when water consumption reaches a peak;
- the late-season stage, when the maturing crop once again requires less water.

Table 10 contains crop factors for the most commonly crops grown under water harvesting.

Table 10 - CROP FACTORS (Kc)

Crop	Initial stage	(days)	Crop dev. stage	(days)	Mid-season stage	(days)	Late season	(days)	Season average.
Cotton	0.45	(30)	0.75	(50)	1.15	(55)	0.75	(45)	0.82
Maize	0.40	(20)	0.80	(35)	1.15	(40)	0.70	(30)	0.82
Millet	0.35	(15)	0.70	(25)	1.10	(40)	0.65	(25)	0.79
Sorghum	0.35	(20)	0.75	(30)	1.10	(40)	0.65	(30)	0.78
Grain/small	0.35	(20)	0.75	(30)	1.10	(60)	0.65	(40)	0.78
Legumes	0.45	(15)	0.75	(25)	1.10	(35)	0.50	(15)	0.79
Groundnuts	0.45	(25)	0.75	(35)	1.05	(45)	0.70	(25)	0,79

Table 10 also contains the number of days which each crop takes over a given growth stage. However, the length of the different crop stages will vary according to the variety and the climatic conditions where the crop is grown. In the semi-arid/arid areas where WH is practised crops will often mature faster than the figures quoted in Table 10.

iv. Calculation of ETcrop

While conventional irrigation strives to maximize the crop yields by applying the optimal amount of water required by the crops at well determined intervals, this is not possible with water harvesting techniques. As already discussed, the farmer or agropastoralist has no influence on the occurrence of the rains neither in time nor in the amount of rainfall.

Bearing the above in mind, it is therefore a common practice to only determine the total amount of water which the crop requires over the whole growing season. As explained in section 2.1.4, the crop water requirement for a given crop is calculated according to the formula:

ETcrop = Kc x ETo

Since the values for ETo are normally measured or calculated on a daily basis (mm/day), an average value for the total growing season has to be determined and then multiplied with the average seasonal crop factor Kc as given in the last column of Table 10.

Example: Crop to be grown: Sorghum - length of total growing season: 120 days (sum of all 4 crop stages according to Table 10) - ETo: average of 6.0 mm/day over the total growing season (from measurement, calculation or Table 9) Crop water Requirement: ET crop = kc x Eto ET crop = 0.78 x 6 = 4.68 mm per day ET crop = 4.68 x 120 days = approx. 560 mm per total growing season

2.2 Water requirements of trees, rangeland and fodder

2.2.1 Multipurpose trees
2.2.2 Fruit trees
2.2.3 Water requirements of rangeland and fodder

2.2.1 Multipurpose trees

There is little information available about the water requirements of multipurpose trees planted under rainwater harvesting systems in semi-arid areas. In general, the water requirements for trees are more difficult to determine than for crops. Trees are relatively sensitive to moisture stress during the establishment stage compared with their ability to withstand drought once their root systems are fully developed. There is no accurate information available on the response of these species, in terms of yields, to different irrigation/water regimes.

Table 11 gives some basic data of multipurpose trees often planted in semi-arid areas. The critical stage for most trees is in the first two years of seedling/sapling establishment.

Table 11 - NATURALLY PREFERRED CLIMATIC ZONES OF MULTIPURPOSE TREES

	Semi-arid/marginal 500-900 mm rain	Arid/semi-arid 150-500 mm rain	Tolerance to temporary waterlogging
Acacia albida	yes	yes	yes
A. nilotica	yes	yes	yes
A. saligna	no	yes	yes
A. senegal	yes	yes	no
A. seyal	yes	yes	yes
A. tortilis	yes	yes	no
Albizia lebbeck	yes	no	no
Azadirachta indica	yes	no	some
Balanites aegyptiaca	yes	yes	yes
Cassia siamea	yes	no	no
Casuarina equisetifolia	yes	no	some
Colophospermum mopane	yes	yes	yes
Cordeauxia edulis	no	yes	?
Cordia sinensis	no	yes	?
Delonix elata	yes	no	?
Eucalyptus camaldulensis	yes	yes	yes
Prosopis chilensis	yes	yes	some
Prosopis cineraria	yes	yes	yes
Prosopis juliflora	yes	yes	yes
Ziziphus mauritiana	yes	yes	yes

Plate 3 - Prosopis cineraria from India

Table 11 is based on the ICRAF Publication "Agroforestry in Dryland Africa", Rocheleau et al. (1988).

2.2.2 Fruit trees

There are some known values of water requirements for fruit trees under water harvesting systems - most of the figures have been derived from Israel. Table 12 contains the water requirements for some fruit trees.

Table 12 - FRUIT TREE WATER REQUIREMENTS

Species	Seasonal water requirement	Place	Source
Apricots	550 mm*	Israel	Finkel (1988, quoting Evanari et al.)
Peaches	700 mm*	Israel	Finkel (1988, quoting Evanari et al.)
Pomegranate	265 mm	Israel	Shanan and Tadmore (1979)
Jujube (Zixiphus mauritiana)	550-750 mm	India	Sharma et al. (1986)

* This figure is the full irrigation rate. Where there was no irrigation but only rainwater harvesting the equivalent of 270 mm depth was adequate to support the trees.

Plate 4 - Jujuba fruits

2.2.3 Water requirements of rangeland and fodder

Water requirements for rangeland and fodder species grown in semi-arid/arid areas under WH systems are usually not calculated.

The objective is to improve performance, within economic constraints, and to ensure the survival of the plants from season to season, rather than fully satisfying water requirements.

2.3 Soil requirements for water harvesting

2.3.1 Introduction
2.3.2 Texture
2.3.3 Structure
2.3.4 Depth
2.3.5 Fertility
2.3.6 Salinity/sodicity
2.3.7 Infiltration rate
2.3.8 Available water capacity (AWC)
2.3.9 Constructional characteristics

2.3.1 Introduction

The physical, chemical and biological properties of the soil affect the yield response of plants to extra moisture harvested. Generally the soil characteristics for water harvesting should be the same as those for irrigation.

Ideally the soil in the catchment area should have a high runoff coefficient while the soil in the cultivated area should be a deep, fertile loam. Where the conditions for the cultivated and catchment areas conflict, the requirements of the cultivated area should always take precedence.

The following are important aspects of soils which affect plant performance under WH systems.

2.3.2 Texture

The texture of a soil has an influence on several important soil characteristics including infiltration rate and available water capacity. Soil texture refers to its composition in terms of mineral particles. A broad classification is:

a. Coarse textured soils - sand predominant - "sandy soils
b. Medium textured soils - silt predominant - "loamy soils"
c. Fine textured soils - clay predominant - "clayey soils"

Generally speaking it is the medium textured soils, the loams, which are best suited to WH system since these are ideally suited for plant growth in terms of nutrient supply, biological activity and nutrient and water holding capacities.

2.3.3 Structure

Soil structure refers to the grouping of soil particles into aggregates, and the arrangement of these aggregates. A good soil structure is usually associated with loamy soil and a relatively high content of organic matter. Inevitably, under hot climatic conditions, organic matter levels are often low, due to the rapid rates of decomposition. The application of organic materials such as crop residues and animal manure is helpful in improving the structure.

2.3.4 Depth

The depth of soil is particularly important where WH systems are proposed. Deep soils have the capacity to store the harvested runoff as well as providing a greater amount of total nutrients for plant growth. Soils of less than one metre deep are poorly suited to WH. Two metres depth or more is ideal, though rarely found in practice.

2.3.5 Fertility

In many of the areas where WH systems may be introduced, lack of moisture and low soil fertility are the major constraints to plant growth. Some areas in Sub-Saharan Africa, for example, may be limited by low soil fertility as much as by lack of moisture. Nitrogen and phosphorus are usually the elements most deficient in these soils. While it is often not possible to avoid poor soils in areas under WH system development, attention should be given to the maintenance of fertility levels.

2.3.6 Salinity/sodicity

Sodic soils, which have a high exchangeable sodium percentage, and saline soil which have excess soluble salts, should be avoided for WH systems. These soils can reduce moisture availability directly, or indirectly, as well as exerting direct harmful influence on plant growth.

2.3.7 Infiltration rate

The infiltration rate of a soil depends primarily on its texture. Typical comparative figures of infiltration are as follows:

	mm/hour
sandy soil	50
sandy loam	25
loam	12.5
clay loam	7.5

A very low infiltration rate can be detrimental to WH systems because of the possibility of waterlogging in the cultivated area. On the other hand, a low infiltration rate leads to high runoff, which is desirable for the catchment area. The soils of the cropped area however should be sufficiently permeable to allow adequate moisture to the crop root zone without causing waterlogging problems. Therefore, the requirements of the cultivated area should always take precedence.

Crust formation is a special problem of arid and semi-arid areas, leading to high runoff and low infiltration rates. Soil compaction as a result of heavy traffic either from machinery or grazing animals could also result in lower infiltration rates.

2.3.8 Available water capacity (AWC)

The capacity of soils to hold, and to release adequate levels of moisture to plants is vital to WH. AWC is a measure of this parameter, and is expressed as the depth of water in mm readily available to plants after a soil has been thoroughly wetted to "field capacity". AWC values for loams vary from 100-200 mm/metre. Not only is the AWC important, but the depth of the soil is critical also. In WH systems which pond runoff, it is vital that this water can be held by the soil and made available to the plants.

The AWC has implications for technical design - for example simple calculation demonstrates that even in deep soils (2 metres) with high AWC values (200 mm/metre) there is no point ponding water to depths greater than 40 cm. This quantity when infiltrated is adequate to replenish the soil profile from permanent wilting point to field capacity and any surplus will be lost by deep drainage as well as being a potential waterlogging hazard.

2.3.9 Constructional characteristics

The ability of a soil to form resilient earth bunds (where these are a component of the WH system) is very important, and often overlooked. Generally the soils which should particularly be avoided are those which crack on drying, namely those which contain a high proportion of montmorillonite clay (especially vertisols or "black cotton soils"), and those which form erodible bunds, namely very sandy soils, or soils with very poor structure.