Actual daily evapotranspiration and crop coefficients for an alpine meadow in the Qilian Mountains, northwest China

Abstract

Actual daily evapotranspiration (ET_a) was measured in two weighing micro-lysimeters, from 1 July 2009 to 30 June 2011 on an alpine meadow in the Qilian Mountains, northwest China. The findings showed that the mean daily ET_a in the unfrozen and frozen periods was 2.0 mm and 0.2 mm, respectively. The dominant factor affecting ET_a in the unfrozen period was net radiation (RN), whereas those in the frozen period were soil surface temperature (T_s) and air temperature (T_a). The mean value of the daily crop coefficient (K_c) was 0.82 in the unfrozen period, and 0.19 in the frozen period. Regression analysis of K_c and the environmental variables indicated that T_a, T_s, relative humidity and soil surface water content (SWC), rather than RN, were the major factors influencing K_c throughout the whole measurement period and in the unfrozen period, although no clear correlative relationships were found between K_c and meteorological factors in the frozen period. Three daily empirical ET_a models for an alpine meadow were developed, also utilizing the FAO-56 Penman–Monteith approach to estimate reference evapotranspiration. They all exhibited good performances in the unfrozen period, but none were found suitable for the frozen period.