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

Abstract

Actual daily evapotranspiration (ETa) 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 ETa in the unfrozen and frozen periods was 2.0 mm and 0.2 mm, respectively. The dominant factor affecting ETa in the unfrozen period was net radiation (RN), whereas those in the frozen period were soil surface temperature (Ts) and air temperature (Ta). The mean value of the daily crop coefficient (Kc) was 0.82 in the unfrozen period, and 0.19 in the frozen period. Regression analysis of Kc and the environmental variables indicated that Ta, Ts, relative humidity and soil surface water content (SWC), rather than RN, were the major factors influencing Kc throughout the whole measurement period and in the unfrozen period, although no clear correlative relationships were found between Kc and meteorological factors in the frozen period. Three daily empirical ETa 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.