Contributions of precipitation, irrigation and soil water to evapotranspiration in (semi)-arid regions

Evapotranspiration (ET) is the largest mode of water loss in cultivated (semi)-arid regions. Isolating the fractions of ET contributed by various components of the water budget is critical not only for water resources management, but also for sustainable crop production, food security and social stability. This study quantifies the fractions of ET from precipitation, irrigation and soil water in the North China Plain (NCP) - a major crop production base in China. About 24 consecutive years (288 months) of hydroclimatic data (spanning from January 1985 to December 2008) are used in the study. The data are derived from Global Land Data Assimilation System (GLDAS) input/output products and field-measured data in the region. Error analysis suggests that uncertainties/biases in the data products are low, with good correlations among the data products. In the NCP study area, precipitation is the highest contributor to ET (39.0%, 318.2 mm), followed by soil water (36.3%, 296.2 mm) and then irrigation (24.7%, 201.5 mm). This is respectively the equivalent of 29.7, 43.9 and 66.4% of average soil water, irrigation and precipitation in the region. Precipitation (as the highest contributor) drives the overall trend, amplitude and phase of ET in the study area. While the contribution of precipitation is highest in the wet summer cropping season, the contributions of irrigation and soil water are highest in the relatively dry spring cropping season. It is vital to devise more efficient ET-reduction strategies to ensure sustainability in water use, food security, ecological protection and social stability in the region and beyond.