A soil-water-balance approach to quantify groundwater recharge from irrigated cropland in the North China Plain

Rapidly depleting unconfined aquifers are the primary source of water for irrigation on the North China Plain. Yet, despite its critical importance, groundwater recharge to the Plain remains an enigma. We introduce a one-dimensional soil-water-balance model to estimate precipitation- and irrigation-generated areal recharge from commonly available crop and soil characteristics and climate data. To limit input data needs and to simplify calculations, the model assumes that water flows vertically downward under a unit gradient; infiltration and evapotranspiration are separate, sequential processes; evapotranspiration is allocated to evaporation and transpiration as a function of leaf-area index and is limited by soil-moisture content; and evaporation and transpiration are distributed through the soil profile as exponential functions of soil and root depth, respectively. For calibration, model-calculated water contents of I I soil-depth intervals from 0 to 200 cm were compared with measured water contents of loam soil at four sites in Luancheng County, Hebei Province, over 3 years (1998-2001). Each 50-m(2) site was identically cropped with winter wheat and summer maize, but received a different irrigation treatment. Average root mean-squared error between measured and model-calculated water content of the top 180 cm was 4.2 cm, or 9.3% of average total water content. In addition, model-calculated evapotranspiration compared well with that measured by a large-scale lysimeter. To test the model, 12 additional sites were simulated successfully. Model results demonstrate that drainage from the soil profile is not a constant fraction of precipitation and irrigation inputs, but rather the fraction increases as the inputs increase. Because this drainage recharges the underlying aquifer, improving irrigation efficiency by reducing seepage will not reverse water-table declines. Copyright (C) 2003 John Wiley Sons, Ltd.