CANAL WATER MANAGEMENT

Water is under stress with ever growing population, particularly in developing countries with high population growth rates. Recent estimates of the World’s hungry population are 923 million in year 2007 (FAO, 2008). It is expected to be further aggravated as there will be additional 2 billion people by the year 2030 (Gany, 2006). The increased population will enhance the global demand for food accordingly, necessitating efficient management of the irrigated agriculture. To cater for food of the population in 2025, it is estimated that water diversions for irrigation need to be enhanced by 14% to 17% and the food production from the irrigated land needs to be enhanced by 40% (Bos et al., 2005).

Despite the facts that Pakistan has the largest contiguous irrigation system in the world, and that the country has predominantly an agrarian based economy, the conditions in Pakistan are not very promising in terms of meeting the food and fiber demands of its 160 Million population. About 20-34% of its population is still suffering from malnutrition (FAO, 2008). Major causes of lack of food are water shortages due to limited water availability in the system and in-efficient use of the available water (Laghari et al., 2008). Recent years have witnessed shortages of irrigation water of up to 40 % of canal withdrawals. The situation is even worsened due to stalled development of large reservoirs since 1976 (the year of completion of the Tarbela dam). System managers are aware of the situation and started to modernize the century old irrigation system. The basic motivation of the modernization is to rehabilitate the old infrastructure and to enhance the efficiencies to the levels being achieved in other parts of the world. Upper Chenab Canal (UCC) is one of the major canal irrigation systems in Pakistan, facing similar problems of low water application efficiencies, low crop yields and deteriorating irrigation infrastructure. FAO (2002) defines modernization as “a process of rehabilitation of irrigation systems during which substantial modifications of the concept and design are made to take into consideration the changes in techniques and technology and to adapt the irrigation systems to the future requirements of operation and maintenance”. It also requires that the delivery of water should be made as flexible as possible with “demand irrigation being the ideal solution”. First step in this modernization is an in-depth diagnosis of the present performance of the system (FAO, 2002) and to understand the agricultural water demands in temporal and spatial domains (Yoo et al., 2008). The present study will complement the modernization drive in the country.

Historically irrigation water has been applied based on water availability (usually in flood seasons) rather based on crop requirement. Design concept of canal system under study is also the same. Developments of new scientific techniques have enabled the mankind to relate the irrigation demands based on physical parameters such as temperature and evaporation. Several methods can be cited in literature ranging from temperature based methods (such as Blaney Criddle formula), pan evaporation based methods (Hargreaves Class A pan evaporation method) to more complex methods such as radiation-resistance based methods (Penmann-1948 method and its subsequent modifications). Allen (1986) has compared the radiation based methods with the lysimeter readings and found that the Penman-Monteith resistance based model provided the most reliable and consistent daily estimates.

Presently the state of the art methods for estimation of reference evapotranspiration and the crop evapotranspiration are the radiation-cum-resistance based methods and have been recommended by Food and Agriculture Organization (FAO, 1998) and by ASCE’s Task Committee on Standardization of Reference Evapotranspiration (ASCE-ET) (Allen et al, 2005). Both FAO and ASCE-ET utilized Penman-Monteith equation, with a difference of reference crops in each of them. FAO favors a standardized grass of 12 cm heights and ASCE has recommended one short crop (grass) and a tall crop (alphalpha) as the reference crops. Although use of the above two methods in various parts of the World is cited (Allen et al, 2005), but only a couple of studies (Ullah et al., 2001; Laghari et al., 2008) are found for Pakistan region employing this scientific method for estimating evapotranspiration and thus the crop water requirement. Laghari et al. (2008) study does not cover a whole canal system rather is limited to a few wheat fields, while the Ullah et al. (2001) estimated crop water requirements for whole of the Indus Basin Irrigation system. Ullah et al. (2001), however, fell short of estimating the irrigation demands and comparing the estimated irrigation demands with the actual supplies. Present study will fill this gap of estimating the crop water requirement based on ASCE standardized Penman-Monteith (2000) equation, converting it to irrigation demands and then comparing it with the actual water supply for having an insight into possible improvement of a canal irrigation system in Pakistan.