Using a basin-scale hydrological model to estimate crop transpiration and soil evaporation

Increasing populations and expectations, declining crop yields and the resu lting increased competition for water necesitate improvements in irrigation management and productivity. A key factor in defining agricultural product ivity is to be able to simulate soil evaporation and crop transpiration. In agribusiness terms, crop transpiration is a useful process while soil and open-water evaporations are wasteful processes. In this study a distributed hydrological model was used to compute daily evaporation and transpiration for a variety of crops and other land covers within the 17,200 km(2) Gediz Basin in western Turkey. The model, SLURP, describes the complete hydrolog ical cycle for each land cover within a series of sub-basins including all dams, reservoirs, regulators and irrigation schemes in the basin. The sub-b asins and land covers are defined by analysing a digital elevation model an d NOAA AVHRR satellite data. In this study, the model uses the FAO implemen tation of the Penman-Monteith equation to simulate soil evaporation and cro p transpiration. The results of the model runs provide time series of data on streamflow at many points along the river system, abstractions and retur n flows from crops within the irrigation schemes and areally distributed so il evaporation and crop transpiration across the entire basin on each day o f an 11 year period. The results show that evaporation and transpiration va ry widely across the basin on any one day and over the irrigation season an d can be used to evaluate the effectiveness of the various irrigation strat egies used in the basin. The advantages of using such a model as compared t o deriving evapotranspiration from satellite data are that the model obtain s results for each day of an indefinitely long period, as opposed to occasi onal snapshots, and can also be used to simulate alternate scenarios. (C) 2 000 Elsevier Science B.V. All rights reserved.