E. Stenitzer, Institute for Soil Water Management, Austrian Federal Water Resources Agency, Petzenkirchen, Austria
The irrigation situation in the semi-arid regions of eastern Austria and practical recommendations for irrigation scheduling with gypsum blocks under these circumstances are briefly described. The need for irrigation is indicated by a 'pilot block' placed 20-40 cm deep with a block reading corresponding to a soil water tension of 0.60 MPa. A second control block at a depth of 60-70 cm signals percolation losses as a result of too high irrigation applications. The farmer is therefore able to properly adjust the irrigation amounts. Selection of a representative site for the gypsum block station within an irrigation field should be based on large scale soil maps, e.g., the Austrian Soil Taxation Map 1:2 500.
Irrigation scheduling by monitoring soil water tension has quite a long tradition (Thorne and Peterson, 1959); tensiometers have been found particulary useful where it is desireable to maintain a plentiful supply of available water for plant growth, while gypsume blocks are normally less sensitive in the high moisture range below 0.1 Mpa tension. This problem has been solved by the development of so-called granular matrix sensors (GMS) as substitutes for tensiometers (Eldredge et al., 1993). Gypsum blocks and granular matrix sensors do not interfere with cultivation, they need not be maintained during operation and their readings may be taken by long wires, thus not disturbing the plants and the soil at the measuring place. Through industrial production the blocks are very uniform and give very reliable data on the soil water potential either directly or in relative units, which can be easily interpreted by a farmer.
Measuring the soil water potential and using it as an indicator for irrigation means one does not have to know anything about the amounts of water in the soil, the percentage of available soil moisture or the plant uptake. Only two pieces of information are necessary; the matric potential in the main root zone; and the threshold value of the soilwater tension which should not be exceeded to protect the plants from water shortage. Fundamental work by Richards and Marsh (1961) and Taylor (1965) gives valuable instructions for the practical utilization of this principle, and more recently Richardson and Mueller-Beilschmid (1989) give detailed information on equipment and costs as well as on field installation, irrigation scheduling and interpreting the field data, showing examples from the arid western states of the United States.
This paper presents a brief description of the recommendations developed by our institute for using the gypsum block method in irrigation scheduling in the semi-arid regions of eastern Austria, including some illustrative examples.
MATERIAL AND METHOD
Irrigation situation in Austria
The most important irrigation areas of Austria are situated in the semi-arid Pannonian region of eastern Austria with cold winters and hot, dry summers. The mean rainfall during the main vegetation period (May to September) typically amounts to 250 - 300 mm, which is significantly less than the water requirement of summer crops such as vegetables, corn and sugar beet. Because of large year to year variations in rainfall, efficient use of irrigation water must be based on the actual soil moisture situation, which integrates the effects of soil quality, plant variables and climate. There is also great need to prevent infiltration of irrigation water into the gravelly subsoils in order to avoid leaching nitrate and pesticides down to the groundwater.
The most irrigated fields are relatively small and can be fully watered within a few days with automatic travelling 'rain guns' which draw their water exclusively from the groundwater. Therefore, irrigations need not follow a fixed time schedule but can be adopted to the actual water availability, i.e., the soil water tension within the rooting depth.
The typical soils in the irrigation region of eastern Austria are silty loams of variable depth with gravelly underground material and a groundwater level about 5-6 m and even more below soil surface. From extensive field research on the soil water balance of these well structured and densely rooted soils it is known that, as a general rule, the soil water tension within the root zone may rise up to 0.4-0.6 MPa without affecting crop yield (Klaghofer and Stenitzer, 1987). Therefore, gypsum blocks are best suited for irrigation scheduling under these circumstances, while tensiometers and even granular matrix sensors should not be used because of their low measuring range.
Testing of the gypsum blocks
The gypsum block method has been tested routinely for more than 10 years during our research on the soil water balance of field crops. In the laboratory the different block types were calibrated in a pressure chamber. The influence of temperature upon block resistance was tested at different ambient temperatures ranging from 5 to 35° C by reading blocks, which had different moisture contents and which were encapsuled to prevent evaporation. The performance of the blocks in the field was checked by comparing water content measurements deduced from gypsum blocks with known soil water contents that were measured by the gravimetric method, by the neutron moisture meter, as well as by time domain reflectrometry.
In elaborating the recommendations it was taken into account that most commercially available hand-held meters do not measure directly in terms of soil water potential but use a dimensionless relative scale ranging between 0-100 units, where the low figures correspond to dry soil conditions, while high figures show that plentiful moisture is available. Hence, different gypsum block meters were compared as well as different types of gypsum blocks; this was done by calibration of different types of gypsum blocks in a pressure chamber, using different types of meters for reading the block resistance.
Introduction of irrigation scheduling with gypsum blocks to the farmers
In order to demonstrate the usefulness of the method, as well as to determine the appropriate threshold value of the soilwater tension, simple irrigation experiments were implemented in close cooperation with farmers and extension services, consisting of different types of irrigation schemes as well as an unirrigated variant.
Furthermore some farmers who were thought to be the opinion leaders in their villages were provided with gypsum blocks and meters to use them for some years on their irrigation fields. The optimal site of the gypsum block station within these fields was chosen on the basis of the Austrian Soil Taxation Map, This map is available for all agricultural lands in Austria at a scale of 1:2 500 giving information on estimated soil texture, soil structure, content of organic matter and rooting density in each soil horizon down to 1.0 m depth.
The results of the tests on the reliability and performance of the gypsum block method were very satisfactory and are described in more detail by Stenitzer (1990, 1993), while this present paper is restricted to the description of some practical results which may be important for the scheduling method to be adopted by the farmers.
Comparison of different meters and block types
The comparison between the various commercially available gypsum block meters showed that all of them had nearly the same interdependence between block resistance and meter readings. Therefore, our recommendations were focused on the differences between the various commercially available gypsum blocks which are shown in Figure 1.
FIGURE 1 - Regression between soil wafer potential and block resistance for the different types of gypsum blocks; the shaded area between 0.20 and 0.60 MPa indicates the region of soil water potential which has been found to be relevant for irrigation scheduling in eastern Austria
The regressions between soil water potential and the sensor resistances were similar at least in that region of soil water tension which is relevant for the indication of irrigation needs of most field crops in eastern Austria: a meter reading of 20 units corresponds to soil water tensions ranging from 0.20-0.24 MPa, depending on the block type used. A meter reading of 10 units indicates a soil water tension of 0.33 MPa, when using a WATERWISE block, while a DELMHORST block will show this reading at a soil tension of about 0.60 MPa, which is still within the range of permissible soil water tension mentioned above.
Irrigation scheduling experiments
These experiments showed that the soil water tension within a depth of 20-40 cm below soil surface should be allowed to rise up to 0.50-0.60 MPa (Figure 2) in order to save water while maintaining a high level of crop yield.
In practical irrigation scheduling, gypsum block readings at this depth should fall to 5-10 points (depending on the block type, see Figure 1) before irrigation is started. Following this instruction, irrigation may not be necessary to achieve economic yields on deep soils with high water storage and frequent rains. This is shown in Figure 3 where the readings of the gypsum blocks at 30 cm depth of an irrigated and a non-irrigated experimental field are compared.
FIGURE 2 - Irrigation scheduling experiment on sugar beet: mean soil water tensions in 20-40 cm depth (from personal communication by F. Karpf, Agricultural Extension Service of Lower Austria)
The irrigated field received 100 mm water and gave an extra yield of 900 kg/ha, which meant an extra income of about 2 100 Austrian Schillings per hectare, while irrigation of 100 mm costs about 5 000 Austrian Schillings per hectare according to official irrigation cost accountings.
FIGURE 3 - Irrigation experiment with maize at Fuchsenbigl (from personal communication by J. Hinterholzer, Federal Agency of Agriculture)
In accordance with these findings we recommend using at least two gypsum blocks per measuring station: one 'pilot block' being situated at about half the depth of the main root zone, to indicate the irrigation needs, while the second 'control block' is placed at the lower boundary of the main root zone and signals percolation losses when meter readings pass 80-90 points, depending on block type.
The gypsum block method as such was well accepted by the farmers as may be demonstrated with Figure 4, which is the copy of the graphical record of gypsum block readings in a sugar beet field on deep and fertile soil. It shows that the farmer scheduled the irrigations during the main growth period from the end of June to the end of August according to our recommendations: irrigation was started whenever the readings of the 'pilot block' at 30 cm depth fell to 10 points. Irrigation height varied between 25 and 40 mm and did not moisten the soil at 60 cm depth during the main growth period as can be seen by the course of the readings of the 'control block' at this depth.
DISCUSSION AND CONCLUSIONS
The few examples given above show that irrigation scheduling by monitoring soil water tension may help the farmer to save water, so lowering irrigation costs and protecting the groundwater as well. The gypsum block method proved to be a simple but reliable method which is accepted by the practical irrigation farmer.
FIGURE 4 - Sugar beet irrigation scheduling by a practical farmer
Experience so far has shown that the gypsum block method is qualified for irrigation scheduling of most field crops under the circumstances described above, i.e., when relatively small irrigation fields of a few hectares may be irrigated within a few days and irrigation water is available at all times. The most difficult problem is to determine the location for the gypsum block station, which should be representative for the whole irrigated field. Soil maps of field scale should be available for this task; otherwise some practical experience must be gained by using several gypsum block stations within the same field.
The recommendations given above may not be useful for irrigation scheduling on sandy soils or for some vegetables and crops that have a rather weak rooting system. In such cases the threshold value of the soil water tension may be lower than 0.20 MPa and the depth of the 'pilot block' may be restricted to 10 - 20 cm below soil surface: in such cases WATERMARK granular matrix sensors could be used.
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