绿洲荒漠过渡带风况对波文比和蒸散发的影响

波文比-能量平衡法是目前蒸散发研究常用的方法之一。在利用波文比-能量平衡法对荒漠地区,尤其对绿洲荒漠过渡带的蒸散发进行研究时,发现有波文比波动大、蒸散发结果准确性不高等问题。如何提高波文比-能量平衡法在绿洲荒漠过渡带的准确性成为荒漠干旱区精确研究水分收支急需解决的问题。在以前的研究基础上,发现绿洲荒漠过渡带的不同风况对波文比有不同影响,这可能是造成波文比-能量平衡法在绿洲荒漠过渡带精度不高的主要原因。为了证明这个假设,于2010—2012年在西北民勤绿洲荒漠过渡带进行了野外连续观测,观测了不同风况条件下波文比、不同高度的温度差、湿度差以及蒸散发的变化规律,分析了它们的特征及其影响因素。结果表明,研究区观测期内波文比值在-17.3—16.2范围内变化,波动较大,呈"U"型变化,生长季中期波文比值低于初期和末期;在无风天气,绿洲荒漠过渡带温度、湿度梯度不受水平气流的影响,波文比波动小,异常值少,波文比方法测算出的蒸散量较为准确,能够代表实际蒸散量,所测得民勤绿洲荒漠过渡带波文比日均值为0.07,日蒸散量为1.6mm/d;在绿洲风和荒漠风天气,绿洲荒漠过渡带空气温度、湿度结构发生明显改变,波文比波动大,不同风况使得大气处于逆温或逆湿的状态,波文比-能量平衡法所测得的蒸散量负值增多,适用性降低。因而在应用波文比-能量平衡法估算绿洲荒漠过渡带的蒸散发时应该选择合适的天气和观测点,避开风况对观测结果的影响。

Effects of wind conditions on the bowen ratio and evapotranspiration in an oasis-desert ecotone

The Bowen ratio-energy balance method is one of the most commonly used methods for estimating evapotranspiration. However, the Bowen ratio can fluctuate greatly and reduce the accuracy of estimates when it is applied to the study of evapotranspiration in desert areas, especially at an oasis-desert ecotone. Improving the precision of the Bowen ratio-energy balance method has become a major focus of studies aiming to accurately estimate water budgets in arid areas. In this study, we found that wind conditions had obvious effects on the Bowen ratio at an oasis-desert ecotone. This may be the main cause of low precision estimates from the Bowen ratio-energy balance method when used at oasis-desert ecotones. In order to test this hypothesis, we recorded meteorological data from May 2010 to June 2012 through the Bowen ratio system at an oasis-desert ecotone in Minqin, China. We analyzed variation in the Bowen ratio, temperature gradient, humidity gradient, and evapotranspiration between two different heights on windy days. The Bowen ratio was lower in the middle of the growing season than at the beginning or end of the season and fluctuated wildly between -17.3 and 16.2. On windless days, horizontal airflow had little effect, causing small fluctuations in the Bowen ratio, and few abnormal values. Thus, the accuracy of evapotranspiration estimates using the Bowen ratio-energy balance method was high. The daily Bowen ratio calculated by the method was 0.07, and the daily evapotranspiration estimate was 1.6 mm/d on windless days. However, the temperature gradient and humidity gradient changed significantly on windy days. Temperature and humidity inversion led to large fluctuations in the Bowen ratio, and more negative evapotranspiration estimates. In these conditions, the accuracy evapotranspiration estimates using the Bowen ratio-energy balance method was reduced. Therefore, when using the Bowen ratio-energy balance method to estimate evapotranspiration at oasis-desert ecotones, it is important to choose the right measurement points and weather to avoid adverse impacts of wind.