Modeling seasonal furrow irrigation

A seasonal furrow irrigation model consisting of submodels to predict irrigation schedule (water balance), irrigation design (surface irrigation hydraulics), and crop yield (yield function) under spatially and temporally variable conditions was developed and verified with the field data. The model was used to predict irrigation performance for each irrigation event during the season, soil moisture before each irrigation, seasonal evapotranspiration (ET), and bean yield along the furrow at 10-m intervals. In addition, measured inputs along the furrow, including heterogeneous infiltration, soil moisture, and yield were used in conjunction with the model to estimate mean (17.3%) and variation in available water-holding capacity (AWC) (15.4%-19.8%). Variation in crop yield represents an integrated effect of variability in infiltration, soil water characteristics, root depth, soil fertility, microclimate, fertilizer and pesticide applications, plants, and disease. Using the calibrated model, nearly 88% of the variance in ET estimated with the yield function was explained by simulated variation in infiltration characteristics, soil water-holding capacity, and root depth. By assuming homogeneous infiltration characteristics, soil water properties, and root depth, the average absolute error in seasonal ET increased by only 0.6 cm but the variance explained decreased to 30%. Although in absolute terms the impact of heterogeneity seems negligible, Raghuwanshi (1994) showed that heterogeneity strongly affected the optimal flow rate and irrigation time as well as net returns to water.