半干旱区不同类型土地利用的蒸散量及水分收支差异分析——以通榆为例

利用吉林通榆半干旱区农田站和退化草地站2008年的外场试验观测资料，对比分析了不同土地利用方式对蒸散和地表水分收支的影响。结果表明：从全年来看，尽管两个站点相距仅5 km，但农田站的全年总蒸散量比代表自然土地覆盖状况的退化草地站高28.2 mm；且生长季两种下垫面的蒸散量较为接近，差异主要发生在非生长季。同时，农田站的年水分收支总量为51.1 mm，比退化草地站低35.6％。具体来说，生长季，两个站点的水分收支均有盈余；但在非生长季，退化草地站的水分收支仍有盈余，而农田站则处于水分亏损状态。这说明在半干旱区，代表人为土地利用状况的农田站面临着更大的水分供给压力，人类活动导致的土地利用会加剧该地区的干旱化趋势。
　　进一步的分析表明，水分盈余并不代表地表的水分状况良好。从 Priestley-Taylor 系数来看，两个站点的Priestley-Taylor系数均远小于1.0，说明在半干旱区，由于表层土壤水分条件的限制，实际蒸散量远未达到平衡蒸散量，土壤面临着水分供给的压力。其可能的原因是，对半干旱区而言，尽管水分收支有盈余，但是由于土壤沙化严重，土壤孔隙度大，大气降水很容易下渗，并以地下水的形式存储起来，使得表层土壤水分供应反而不足。

Analysis of Evapotranspiration and Water Budget for Various Land Use in Semi-Arid Areas of Tongyu, China

Observational data from Tongyu cropland and degraded grassland sites recorded from January to December 2008 were used to investigate the effects of land use and cover changes on evapotranspiration and surface water budget. The results show that although the two sites are only 5 km apart, the annual total evapotranspiration of the cropland ecosystem was 28.2 mm higher than that of the degraded grassland ecosystem. This difference occurred mainly in the non-growing season. In addition, the annual total water budget of the cropland was determined to be 51.1 mm, which is 35.6% lower than that of the degraded grassland. In the growing season, the water budgets of both sites were positive. Although the water budget in the degraded grassland remained positive during the non-growing season, that in the cropland was negative. These results indicate that the cropland ecosystem, which involves a higher extent of human activities, is affected by more severe water supply issues in semi-arid areas and that land usage changes introduced by human activities aggravate aridity trends. Moreover, further analysis suggests that a positive water budget does not necessarily guarantee that the land surface includes an ample water supply. The Priestley-Taylor coefficients of both sites were determined to be far less than 1.0, which indicates that the actual evapotranspiration value did not reach the equilibrium evapotranspiration value due to limited water availability at the land surface. Therefore, the surface soil of the semi-arid area was adversely affected by an insufficient water supply. This result is illustrated by the soil texture of the semi-arid area, which is characterized by severe erosion and high porosity. Under such conditions, although the water budget is positive; however, the precipitation is likely to infiltrate to deeper soil layers that serve as groundwater storage, eventually resulting in water shortage at the shallow layer of soil.