The Contribution of Root Accessible Water Tables Towards the Irrigation Requirements of Crops
Irrigation world wide leads to increases in groundwater recharge, which in turn result in rising water tables. South Africa is no exception since it is estimated that shallow water tables, in or just below the potential rooting depth of annual crops, can be found in at least 20% or approximately 260 000 ha of irrigated soils. Proper utilization of shallow water tables can contribute significantly, through capillary rise in the root zone, towards water requirements of crops and is recognized as an important water resource in agriculture. When utilized improperly, however, shallow water tables can result in severe crop and soil losses due to salinization of the upper part of the root zone.
The quantification of capillary contributions from shallow water tables towards crop water requirements is therefore considered to be a very important management tool, to ensure conservation of water and soil resources. This, together with observed crop losses caused by waterlogging and increased soil salinity, motivated this investigation. The objectives of this research project are to:
A three year lysimeter experiment was conducted with a yellow sandy Clovelly Setlagole soil and a red sandy loam Bainsvlei Amelia soil to quantify the contribution of root accessible water tables towards the water use of potatoes, wheat, maize, peas and groundnuts. Five treatments, replicated three times, and randomly allocated to the containers of each soil, were applied as follows:
A pertinent effort was made to create, with every treatment, optimum conditions for crop growth and maximum water use, allowing for more accurate quantification of maximum water uptake. Very high seed and biomass yields, confirmed by and related to very high evapotranspiration and water table depletion values, were therefore recorded. Average crop yields of 9507, 15075, 3760 and 3383 kg ha-1; evapotranspiration of 877, 987, 523 and 766 mm; and water use efficiencies of 10.8, 15.3, 7.2 and 4.4 kg ha-1 mm-1 for wheat, maize, peas and groundnuts respectively were attained. Unfortunately, the potato crop was killed by early frost before maturity.
Wheat and groundnuts used more and less water, respectively, from the water tables compared to maize or peas under similar conditions. The cumulative uptake from the water tables over the growing season ranged between 38 and 63% for wheat, 25 and 53% for maize, 30 and 55% for peas and 21 and 45% for groundnuts. During the period of peak water uptake the constant 1 m water table contributed up to 90% of the daily evapotranspiration for all the crops.
Important factors that controlled the plant water uptake were, inter alia, the rate and height of capillary rise from the water tables. Columns with artificial water tables under controlled conditions were therefore used to determine upward flux as a function of height above the water table for a range of soils. The results indicated that the height of capillary rise increased with an accompanying increase in the silt-plus-clay content of a soil. At a specific height above the water table however, the upward flux was higher for soils with higher silt-plus-clay contents. These results were used to develop procedures by which hydraulic conductivity-matric potential and matric potential-volumetric water content relationships could be predicted.
The one dimensional finite difference approach was incorporated into the Soil Water Balance (SWB) model of Annandale et al. (1999), whereas the upward cascading approach was used in the Soil Water Management Program (SWAMP) developed by Bennie et al. (1998). Both models were used to simulate the uptake from shallow water tables by plant roots, and the estimated values were found to be of acceptable accuracy compared to the measured values. Any of these two models can therefore be recommended to predict the contribution of water tables to crop water uptake for a variety of crops, soils and water table depths.
From this investigation it was evident that the irrigation requirements of crops could be reduced with the amount of water taken up from a water table and consequently, procedures were developed to modify irrigation scheduling methods for such conditions. For the water balance based method of irrigation scheduling either the SWB or SWAMP computer models can be used to predict the water table uptake by crops, and these estimated values can be subtracted from the normal irrigation requirements of crops. For the crop coefficient based method of irrigation scheduling however, factors were derived by which the coefficients of crops can be adjusted for two water table depths and different textural soils. Application of these recommended procedures can result in 30 to 65% reductions in the irrigation requirements of crops with corresponding irrigation water savings.
The presence of shallow water tables is usually an indication that salinization is present to varying degrees in irrigated soils. Future research is therefore needed to quantify the effect of salinization on crop water use. This will allow for the improvement of irrigation scheduling models. The models used, however, for irrigation scheduling in South Africa do not accommodate the effect of salinization on crop water use.