Net radiation rather than surface moisture limits evapotranspiration over a humid alpine meadow on the northeastern Qinghai‐Tibetan Plateau

Abstract

AbstractAccurately quantifying evapotranspiration (ET) is crucial to fully understanding regional water resource management and potential feedbacks to climate change in alpine grasslands. The quantitative relationships between ET and environmental controls were investigated by a continuous eddy covariance dataset from June 2014 to December 2016 over an alpine Kobresia meadow on the northeastern Qinghai‐Tibetan Plateau. The results showed that daily ET averaged 1.7± 1.5 mm/day (Mean ± 1 S.D.), with values of 2.9 ± 1.3, 1.6 ± 1.0, and 0.7± 0.6 mm/day during the growing season, seasonal transition period, and nongrowing season, respectively. Cumulative growing season ET was 63% of annual ET with little annual variability (349.9 ± 12.1 mm). Paired‐samples t‐test analysis indicated that monthly ET was larger than maximum potential ET derived from the FAO‐56 reference crop ET by 17% (p < .001, N = 12) in the growing season, likely because of high aerodynamic conductance, but was less than the minimum equilibrium ET by 19% (p < .001, N = 14) during the nongrowing season owing to limited surface moisture availability from the frozen soil. The structural equation models revealed that daily ET was mostly dominated by net radiation (the standardized coefficient of the total effect was 0.78). Soil surface moisture and leaf area index played secondary roles in daily ET variability during the nongrowing season and growing season, respectively. At an annual scale, the bulk surface conductance (8.25–10.65 mm/s), decoupling coefficient (0.43–0.48, 0.61 in the growing season), and the ratio of ET to equilibrium ET (1.08–1.33) were consistent with the strongly energy‐limited conditions in the alpine meadow. This study indicated that initial vegetation rehabilitation on the severely degraded meadow would be at the risk of rapid water consumption in humid alpine regions.