Pakistan Journal of Meteorology Vol. 6, Issue 11 61 Effective Rainfall for Irrigated Agriculture Plains of Pakistan Shahzada Adnan 1 , Azmat Hayat Khan 1 Abstract Rainwater is essential source for agriculture, human beings and animals that falls on the surface of earth. Agriculture production in Pakistan is highly dependent upon precipitation, which either fall in the form of rain or snow. Not all that is useful rather a part of it is effective. This study deals with “effective rainfall”, i.e. useful rainfall. Agriculturists consider the portion of rainfall effective that directly satisfies crop water requirements. In this study, four different methods have been used to estimate effective rainfall using data from 58 meteorological stations covering irrigated plains of Pakistan. The effective rainfall have been estimated for two crop growing seasons, i.e. Rabi(October to April) and Kharif(May to September). It has been observed that effective rainfall values for Rabi and Kharif season varies widely from 13.03% and 21.31% at humid zone of northeastern Punjab to 100% at several stations by Renfro Equation method, 43% and 30% at humid zone of northeastern Punjab, 99.86% to 100% is at central Sindh by U.S Bureau of Reclamation Method. 17.57% at humid zone of northwestern NWFP, 98.98% and 99.93% at arid zone of southwestern Sindh and Balochistan by Potential Evapotranspiration /Precipitation Ratio method and 54.40% and 60% at arid zone of southern Balochistan to 100 % at several stations by U.S.D.A, SCS method respectively. Murree has the lowest amount of effective rainfall. It has been observed effective rainfall is directly proportional to consumptive use, water storage capacity and irrigation application. Renfro equation is not suitable for short term planning. U.SBR recommended for regions of heavy amount and high intensities of rainfall. PET/ Precipitation Ratio method is most effective for preliminary planning than the rest and U.S.D.A, SCS method is for areas, which receive low intensity of rainfall like southern parts of Pakistan. It has been observed as the distance from the sea increases the value of effective rainfall decreases except northern areas. While designing an irrigation project, effective rainfall can be the most advantageous for utilization of irrigation water. Keywords: Effective rainfall, Agriculture, Agriculturists, Climate, Pakistan Introduction Rainwater is highly important for agriculture as well as human beings and animals that fall on the earth surface. Pakistan is a land where rainfall not only falls in summer but also in winter. During the summer monsoon period (July to September) easterly systems/depressions form in the Arabian Sea and the Bay of Bengal, produce rainfall over low elevation plains of Pakistan. August is the peak month of that season. Nearly 60% of annual rainfall over most parts of Pakistan is received during summer (June to September). Rainfall occurs primarily due to differential heating of the land and sea [3]. Kharif crop largely depends upon the amount and distribution of rain especially during monsoon season. [1] In Pakistan, winter precipitation occurs due to the western disturbances which are generally the off-shores of mid latitude frontal systems. These disturbances move in the northeasterly direction. These disturbances pick up the moisture from Persian Gulf as well as from the Arabian Sea and not only enhance the winter rainfall but also cause rainfall over the most parts of the Pakistan during winter [12]. These rains are very important for Rabi crops in rainfed areas. The primary source of water supply for agriculture in most parts of the world is rainfall. The characteristics of rainfall vary from place to place, day to day, month to month and also year to year. In spite of voluminous data on weather, all is not yet known about rainfall variability over temporal and spatial scale. Certain simple entities have baffled planners right up to present. One of these is “effective rainfall". In its simplest sense, effective rainfall means useful or utilizable rainfall. Rainfall is not necessarily useful or desirable at the time, rate or amount in which it is received. Some of it may be unavoidably wasted while some may even be destructive. Just as total rainfall varies, so does the amount of effective rainfall [6]. The useful portion of rainfall is stored while the rest unwanted parts need to be detached or pass on speedily. 1 Pakistan Meteorological Department