Laserfiche WebLink
<br />.,........ <br />';,.... <br /> <br />'" <br />--..l <br />co <br />00 <br /> <br />102 <br /> <br />APPROACHES TO DRAINAGE PROBLEMS <br /> <br />EMPIRICAL SPECIFICATIONS <br /> <br />The following case study illustrates the concept described above. A river of <br />500units volumeand an initial ECofO.5 dS/m was assumed to pass through four <br />successive identical agricultural projects. In each project, diversions from the <br />river were used to irrigate four crops. Part ofthe applied water is consumed by <br />the crops in evapotranspiration; the rest becomes drainage. Inefficiencies and <br />losses of water in transit are not considered for our purposes. The following <br />amounts were assumed to be consumed in each project for each crop: beans (15 <br />units). alfalfa (40 units), wheat (25 units) and collon (14.4 units). The water <br />volumes applied to each crop were chosen sufficient to meet these ET levels <br />and result in the following leaching fractions: beans (L = 0.2), alfalfa (L = 0.1), <br />wheat (L = 0.2),andcollon (L= 0.15). These values of La reno to ptima I under <br />all conditions, but are representative of reasonably good management. <br />Three different irrigationfdrainage management strategies were simulated. <br />In Strategy I, each crop is irrigated with theriverwater(atitssalinity level when <br />entering each project) and all of the percolated drainage water from each crop <br />is assumed 'to be collected by some drainage system and returned to the river <br />below that project's irrigation diversion point The resultant "blend" of river <br />and drainage water is then assumed.to now on 10 the next downstream project <br />where the process is repeated. This process is repeated through the succession. <br />of four projects. In Strategy II, the bean, alfalfa and wheat crops are also <br />irrigated with the river water; however, their resulting drainage waters are not <br />returned to the river. Instead they are combined and used to irrigate the cotton <br />grown within the same project. The secondary drainage water resulting after <br />cotton production is then returned to the river. This process is also repeated <br />through the succession oUour projects. In Strategy I!I,the crops are irrigated <br />as in Strategy II, but the secondary drainage water from the 'collon is not <br />returned to the river. This relatively small volume of water is assumed to be <br />disposed of by some other means (such as evaporation, desalting, deep-aquifer <br />injection, etc.). <br /> <br />ECONOMIC EVALUATIONS OF ll-tE CASE STUDY RESULTS <br /> <br />The detailed physical results of drainage management Strategies (I, II. and <br />III) are given in tables 1,2, and 3, respectively. Summary results are given in <br />table 4. <br />Compared to Strategy I, less irrigation water is diverted from tlte river and <br />less drainagewater is returned to it under the management practices of Strategy <br />II. Whereas the volume and salinity of the river is the same under both <br />strategies, less yield isoblained in Strategy II. All of this yield loss is accounted <br />