Relationship between crop coefficient and soil evaporation in forage grassland under sprinkler irrigation in Lhasa Valley

Abstract

【Objective】 Crop coefficient and soil evaporation are two important parameters for calculating evapotranspiration in terrestrial ecosystems but they are dependent on many factors. In this paper, we investigate their correlation for a forage grassland. 【Method】 The experiment was conducted in a forage grassland (Triticale and Oat) irrigated by sprinkler irrigation at the Lhasa Valley. Evapotranspiration (ET) was measured from a micro-lysimeter and the potential evapotranspiration was calculated using the PM method. The ratio of the former to the latter was the ground-truth crop coefficient (Kc). This crop coefficient was used to evaluate the single-crop coefficient estimated using the modified PM method. We also measured the inter-plant soil evaporation (E) using an in-house instrument to estimate the E/ET ratio. The relationship between Kcand E/ET, as well as their dependence on leaf area index (LAI) and SPAD were established. 【Result】 Using the correction method recommended by FAO-56, the crop coefficient of Triticale and Oat in the Lhasa valley at initial, middle and end of their growth stage was Kc ini=0.45, Kc mid=1.22 and Kc end=0.49 for the Triticale, and Kc ini=0.94, Kc mid=1.16 and Kc end=0.31 for the Oat. For the Triticale, soil evaporation accounted for 44.8% of the evapotranspiration during the whole growth, and 93.4%, 94.8%, 61.8%, 22.2% and 13.8% during the seedling-overwintering, overwintering-replanting, replanting-jointing, jointing-heading and heading-mowing stage, respectively. For the Oat, soil evaporation accounted for 98.9%, 79.1%, 31.7% and 16.8% of the evapotranspiration from seedling to tillering, tiller to jointing, jointing to heading and heading to mowing stages, respectively. Overall, soil evaporation accounted for more than 44% of the evapotranspiration and it is hence potential to improve water use efficiency by reducing soil respiration. The measured crop coefficient and the ratio of soil evaporation to evapotranspiration were significantly correlated with LAI and SPAD linearly or nonlinearly. 【Conclusion】 The measured crop coefficient, ratio of soil evaporation to evapotranspiration of the two-season forage grassland are significantly correlated to LAI and SPAD. Adding a crop indicator to the single crop coefficient can improve the accuracy of the calculated evapotranspiration. Through mathematical models, the crop indicators can simulate evapotranspiration.

Keywords

• lhasa valley
• sprinkler irrigation
• two-season forage grass
• crop coefficient
• soil evaporatio