WSP11706 - Laserfiche WebLink

Home Browse Search

w W N c.e .; ,; CHAPTER I. INTRODUCTION Proiect Background Rivers draining arid basins increase in salinity content as the flow moves down- stream. In nature, much of the salt content of the water flowing from the mountain basins accumulates in the soil as the waters infil- trate or evaporate. When these lands are used for agriculture, the applied water leaches more salt through the soil and into the river. As river water becomes more saline, its value in agricultural and urban uses de- clines. Andersen et al. (1978) found that in the Colorado River Basin, damages are be- coming major as salinity levels at Imperial Dam pass 1000 ppm and now amount to approxi- mately $20 per ton of salt removed ($200,000 for an average reduction in salt content of 1 ppm). They also found on-farm irrigation water management practices in upstream irrigated areas to be the least expensive salinity control method. Narayanan et al. (1979) also found methods to improve irriga- tion efficiency to be quite promising as salinity control measures. These and the many other studies which could be cited that identify irrigation water management as important for salinity control, however, do not indicate exactly what manage- ment practices should be used. That issue has yet to be resolved through an improved understanding of how salt loadings from irrigated areas vary with farm water manage- ment practices employed. A model is needed to capture the essence of this relationship with mathematical expressions for quantita- tive prediction. Data are needed to calibrate such salinity-control applications. Two very important needs are a good model and good data for calibrating it. In selecting a good model, one must recognize that the options come with wide variation in complexity of equations used, amount of descriptive input information required, and reliability of results pre- dicted. For overall irrigation water manage- ment system design, one needs a tool that can be applied to a fairly large area without requiring an unreasonably extensive amount of data and that is accurate enough to lead to salinity control practices that work. These river basin hydrosalinity models are powerful tools for 1) finding alterna- tives to existin~ water management procedures to improve and control sal.inity levels in irrigation return flows, and 2) predicting future salinity levels at critical reaches within a river system. Such predictions make it possible to assess achievements toward complying with the salinity level require- ments of the Water Pollution Act Amendments of 1972 (PL 92-500), Hydrosalinity modeling uses the mod- eler's present understanding of the various hydrologic and salinity transport processes that occur within an irrigated area to predict how the system as a whole will respond to changes, such as alterations to farm water manag.ement policy, that may directly affect only a few processes. Model predictions depend on the assumptions made in representing the various processes as well as on how the model portrays inter- actions among the processes to produce aggregate results. Hydrologic modeling provides the basic framework for river basin hydrosalinity models, but several key processes must be added, or at least given additional emphasis. Three of the most critical are: 1. The chemical reactions and inter- actions that occur as waters con- taining varying combinations of salt species move, in an irregular time pattern, through soils having a variety of chemical properties. 2. The respective sources and degrees of mixing among surface runoff, natural groundwater, and irrigation return flow. In hydrologic modeling one simply adds these flows and does not need to be so careful in getting the correct mi xture between them because errors can offset. In hydrosalinity modeling, the correct ratios among flows originating from various sources are impor- tant to portraying the chemical reactions correctly. Since gaged stream flow data seldom indicate flow sources, this requirement poses major difficulties for model verfication. 3. Salt pick up in effluent ground- waters is site specific depending upon the presence of residual salts and the extent of mineral dissolu- tion in the groundwater zone. It is important to represent well the processes controlling salt pickup as well as those controlling the depositing and subsequent repeat