Evapotranspiration Distribution and Variation of Pakistan (1931-2015)

Abstract

Abstract Evapotranspiration is the main element of aridity and desertification and to balance the natural hydrological processes. Pakistan has a high degree of evapotranspiration, as it is in subtropical belt, with long sunshine duration and low cloudiness in summers. June is the warmest month, when the evapotranspiration exceeds 7mm (0.28inches), whereas, January is the coldest month, when evapotranspiration of the country falls to 1mm (0.04inches). The maximum evapotranspiration has been recorded at the southern latitudes of the country (Hyderabad and Jacobabad), while it decreases towards northwest (mountainous region) and Gilgit-Baltistan (Astore and Skardu). This variation in evapotranspiration is due to fluctuation in temperature, precipitation, sunshine duration, wind speed, relative humidity, physical relief and latitudinal as well as altitudinal extend of the country. The average evapotranspiration of Pakistan is 4.5mm with an increase of 1.0mm during 1931-2015. In winter and summer season, the lower Indus basin, has recorded high evapotranspiration as compared to the northern mountainous region. The average evapotranspiration of Pakistan during winter season is 2.7mm, while in summer it is 6.3mm. This variation is due to the variation in the length of day and night, humidity, precipitation, surface pressure, wind speed, and topography of the land. During cold season the average evapotranspiration of the country is 13.7mm, pre-monsoon season 17.1mm, monsoon season 15.8mm and post monsoon season 8mm. Obviously, the highest evapotranspiration of Pakistan has recorded during pre-monsoon season with extreme temperature, scarce precipitation, long sunshine duration, lowest relative humidity, low pressure, and calm winds and chilly condition. Furthermore, during cold (0.1mm), pre-monsoon (3.5mm), and monsoon season (2.2mm) the evapotranspiration shows an increase, where as it reveals a negative deviation of -5.6mm in post monsoon season due to increase in the precipitation from reversible monsoon lows at the southern latitudes of the country. Generally, the evapotranspiration of Pakistan increases from northwest to southeast and a main agent of delimitation of the arid region of the country. The main factors that cause variation in the evapotranspiration of the country from south towards north are temperature, precipitation, sunshine duration, relative humidity, surface pressure, wind speed, fogs, cloudiness, topography, latitudinal and altitudinal extend of the country that required further research.