Daily Crop Coefficient of Spring Maize Using Eddy Covariance Observation and Its Actual Evapo-transpiration Simulation

Spring maize is one of the most important crops in Northeast China and accounts for about 1/3 of grain crop area. Due to climate change in Northeast China during recent years, climate warming and drying trend is very significant. As a result, drought disasters of spring maize occur frequently, moreover, it often occurs in the critical period of the formation of maize production, resulting in a serious impact on maize yield. How to scientifically irrigate maize farmland and ensure maize yield stable and high is a serious challenge. In order to accurately calculate the actual evapotranspiration of maize, dynamic daily crop coefficient of spring maize and its relationship with leaf area index are studied, using the latent heat flux data from eddy covariance (EC), and corresponding data including meteorological data, phenological data and leaf area data during 2006-2008 and 2011 at Jinzhou Agricultural Ecosystem Research Station. Results indicate that both daily crop coefficient and actual evapotranspiration of spring maize farmland ecosystem show a unimodal curve change, and they reach the maximum from late July to early August (maize flowering and silk stages). A new dynamic crop coefficient model under conditions of enough water supply is developed for spring maize, and it indicates the close linear relationship between crop coefficient and leaf area index (R²=0.88, F=73.5, P < 0.01). Furthermore, the relative leaf area index is simulated using the standardization of growth period based on cumulative temperature. The relationship between daily crop coefficient of spring maize and relative leaf area index are also developed (R²=0.93, F=527, P < 0.01), which solves the calculation of daily actual evapotranspiration over spring maize farmland ecosystems without the leaf area observation. This new model improves the crop coefficient suggested by FAO, and extends the calculation from phonological stages to daily scale.
    At present, crop coefficients come from different evapotranspiration observation methods, including lysimeter and eddy covariance, and different methods lead to significantly different results. The comparison shows that crop coefficients of maize at four phenological stages based on the evapotranspiration observations from eddy covariance towers are the closest to values suggested by FAO. The newly developed crop coefficient model is able to simulate daily actual evapotranspiration of spring maize farmland ecosystem with a good accuracy. It could provide theoretical basis for the management of agricultural water resources and irrigation.