Estimating Spatially Explicit Irrigation Water Use Based on Remotely Sensed Evapotranspiration and Modeled Root Zone Soil Moisture

Abstract:

Abstract: Irrigation accounts for ~90% of global consumptive freshwater resources. Estimation of irrigation water use is critical to assessing water use efficiency and optimizing water resources allocation. However, various irrigation scheduling methods performed by farmers or districts, together with diversity in irrigation systems and crop types have imposed restrictions on deriving the accurate spatial distribution of irrigation. Furthermore, many hydrological models do not accurately represent the effect of irrigation given data limitation at various scales across different regions. Remote sensing observations could provide valuable information on moisture transport over the cropland, which could unravel the total crop water consumption induced by climate variability and human intervention. In this study, we first develop the relationship between evapotranspiration (ET) fraction (the ratio of actual ET to reference ET) and root zone soil moisture derived from the Community Water Model (CWatM) without the irrigation module during rainfed summer maize period. Second, the root zone soil water content under the irrigation condition was derived from the developed relationship and remotely sensed ET from the two-source energy balance model (TSEB) during irrigated winter wheat period. Finally, irrigation water use was derived by changes in soil water content and actual ET. The estimation of irrigation water use is reliable based on in situ measurements derived from experiment stations over cropland in the North China Plain, with root mean square error of ~80 mm at annual timescale. Integrating remote sensing information with CWatM produces irrigation water use estimates of higher accuracy than those derived solely from CWatM. Meanwhile, the irrigation water use estimates can reflect trends in agricultural water use due to anthropogenic activities in the long term, indicating the variations of water consumption in crop production.

Key words: Irrigation; Community Water Model; Evapotranspiration; Root zone soil moisture