MODELING SEASONAL FURROW IRRIGATION

A seasonal furrow irrigation model consisting of submodels to predict irrigation schedule (water balance), irrigation design (surface irriga tion hydraulics), and crop yield (yield function) under spatially and temporally variable conditions was developed and verified with the fie ld data. The model was used to predict irrigation performance for each irrigation event during the season, soil moisture before each irrigat ion, seasonal evapotranspiration (ET), and bean yield along the furrow at 10-m intervals. In addition, measured inputs along the furrow, inc luding heterogeneous infiltration, soil moisture, and yield were used in conjunction with the model to estimate mean (17.3%) and variation i n available water-holding capacity (AWC) (15.4%-19.8%). Variation in c rop yield represents an integrated effect of variability in infiltrati on, soil water characteristics, root depth, soil fertility, microclima te, 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 infiltratio n characteristics, soil water-holding capacity, and root depth. By ass uming homogeneous infiltration characteristics, soil water properties, and root depth, the average absolute error in seasonal ET increased b y only 0.6 cm but the variance explained decreased to 30%. Although in absolute terms the impact of heterogeneity seems negligible, Raghuwan shi (1994) showed that heterogeneity strongly affected the optimal flo w rate and irrigation time as well as net returns to water.