Development and testing of soil water regime simulation models

Four soil water balance simulation models corresponding to specific soil-crop relations were developed for application to irrigation planning and management. The forms of the models were inferred from 18 months of weekly and bi-weekly soil water data and daily meteorological data. Soil water change is computed by budgeting of the water inputs and outputs, namely precipitation, evapotranspiration, drainage, and runoff. Actual evapotranspiration was found to be dependent on both potential evapotranspiration and soil water content. Empirical drainage functions were developed, but semi-empirical ones inferred from theoretical knowledge of soil hydraulic properties performed at least as well. Runoff functions were required to explain only exceptional conditions of very heavy rainfall. A quantitative assessment of each model's prediction accuracy was performed. The uncertainty that can be expected for any predicted value with a cumulative probability of 0.95 is in all cases within an interval of 1% of the soil water content in average conditions.