New insights on evapotranspiration and water yield in crop and forest lands under changing climate

Abstract

Forest lands are sometimes thought to increase evapotranspiration (ET) and decrease water yield (WYLD) more than those of croplands, especially in the semi-arid subtropical and tropical regions. Using the US-EPA (Environmental Protection Agency)’s Hydrologic and Water Quality System (HAWQS) model along with Mann Kendall statistics (τ) and Kolmogorov-Smirnov test, I compared ET and WLYD between a cropland and a forest land in a humid subtropical region – Yazoo River basin (YRB), Mississippi, USA – under the changing climate. Results show that no temporal trend of ET was found over the past 50-year (1966 to 2015) and future 50-year (2021 to 2070) climate conditions, but a significant increasing trend (τ = 0.403, p < 0.01) of ET was observed in the recent 20 years (1997 to 2017) in both the crop and forest lands. The simulation further reveals that the annual average air temperature over the past and future 50 years was about 1 °C cooler in the forest land than in the cropland. During the past 50 years, there was 10.8% more water lost from ET in the cropland (656 mm/year) than in the forest land (592 mm/year), while there was 17.3% less WYLD in the cropland (711 mm/year) than in the forest land (834 mm/year). Similar results were also obtained for the future 50 years, i.e., there were 42.0% more ET and 20.7% less WYLD in the cropland than in the forest land. Results show that over a long-range, the forest land reduced ET and increased WYLD as compared to those of the cropland in the YRB (a humid subtropical region). These new insights change the traditional views on how forests and crops affect ET and WYLD in the humid subtropical regions around the world and provide useful information to farmers and foresters for effective water resource management