Temporal Downscaling of Crop Coefficients for Winter Wheat in the North China Plain: A Case Study at the Gucheng Agro-Meteorological Experimental Station

Abstract

Abstract:The crop coefficient (K c ) is widely used for operational estimation of actual evapotranspiration (ET a ) and crop water requirements. The standard method for obtaining K c is via a lookup table from FAO-56 (Food and Agriculture Organization of the United Nations Irrigation and Drainage Paper No. 56), which broadly treats K c as a function of four crop-growing stages. However, the distinctive physiological characteristics of overwintering crops, such as winter wheat (Triticum aestivum L.), which is extensively planted in the North China Plain (NCP), are not addressed in this method. In this study, we propose a stage-wise method that accounts for K c variations for winter wheat at each critical phenological stage, thereby estimating K c at finer temporal scales. Compared with the conventional FAO method, the proposed stage-wise method successfully captures the bimodal pattern in K c time series for winter wheat, which is shown at both ten-day and phenological time scales. In addition, the accuracies of the proposed stage-wise K c method and the FAO method were evaluated using micro-meteorological measurements of ET a collected at the Gucheng agro-meteorological experimental station in the NCP. Using a leave-one-out strategy, the evaluation revealed that the stage-wise method significantly outperformed the FAO method at both daily and critical phenological time scales, with root-mean-square errors in ET a for the stage-wise method and the FAO method being 0.07 mm·day −1 and 0.16 mm·day −1 , respectively, at the daily time scale, and 0.01 mm·day −1 and 0.27 mm·day −1 at the critical phenological time scale. Generally, the FAO method underestimates ET a during the initial stage and overestimates ET a during both the development and mid-season stages. It is shown that the proposed stage-wise method is important for the water-stressed NCP where precision irrigation is highly desirable, especially during the critical phenological stages. Results from this study provide insight into accurate estimation of water requirements for winter wheat at phenological time scales.

Spatio-Temporal Pattern Change of Winter Wheat Production and Its Implications in the North China Plain

“…In the NCP, irrigation is an important guarantee to meet water demand for crop growth and production of winter wheat, due to scarce water resources and low precipitation [15,17,20,46]. In the 1960s, the government gave priority to food security, and actively advocated the development of irrigation facilities and implemented free use of irrigation water (only agricultural electricity fee was needed) [37].…”

“…However, winter wheat is the most irrigation demanding crop and consumes more than 50% of the total irrigation water. As wheat production heavily relies on irrigation from groundwater resources [12][13][14][15], the large production in the NCP that was mainly attributed to the expansion of irrigated area and increase of water supply per unit sown area has caused serious depletion of groundwater, especially in the northern NCP with scarce water resources [16][17][18]. During the last decades, the local governments took some measures to relieve this problem by reducing sown area of winter wheat, and thus could induce the spatial pattern changes of winter wheat production and might affect stability of wheat total production in the NCP [12,14,[19][20][21][22][23].…”

Spatially Explicit Modeling of Coupled Water and Carbon Processes Using a Distributed Ecohydrological Model in the Upper Heihe Watershed, China

“…However, the sophistication of individual, and interactions between, hydrological, ecophysiological and energy cycles are hard to comprehend because of the related nonlinear feedback mechanisms, especially for landscape-scaled ecohydrological processes and the uncertainty of natural and anthropogenic disturbances [3]. The tight linkage between water and carbon dynamics is expressed in many hydrologically controlled ecological processes, for instance, respiration [4], photosynthesis [5], water use efficiency [6] and crop coefficient analyses [7].…”