Agricultural drainage water management in arid and semi ... - FAO.org

36Water quality concerns in drainage water managementwater quality, it is necessary tounderstand water movement overand through the soil and theassociated suspended and dissolvedsubstances it carries.FIGURE 14Water flow over and through the soilIrrigation/RainfallEvapotranspirationOf the water added to the soil,either in the form of rainfall orirrigation, part is lost through runoffand direct evaporation at the soilsurface. Runoff water collects innatural and constructed surfacedrains from where it finds its way tothe final disposal site (a river,evaporation pond or outfall drain tothe ocean or saline lake). The otherpart infiltrates into the soil. Thiswater fills up the soil pores andRunoffCapillary risePercolationSubsurfacedrainageWater tableRoot extractionSoil surfaceBottom rootzonerestores the soil moisture content up to field capacity under free drainage. The stored water isnow available for plant root extraction to satisfy the water requirement of the crop. Any waterin excess of field capacity percolates below the rootzone to greater depth in the vadose zone.The deep percolation water may eventually serve as recharge to the groundwater or saturatedzone. In irrigated areas with shallow groundwater tables, the recharge is immediate and causesthe water table to rise. Where subsurface drainage is installed in waterlogged soils, the drainagesystem removes deep percolation and groundwater. Where the soil moisture content in therootzone drops as a result of evapotranspiration and if there is no recharge from irrigation orrainfall, capillary rise into the rootzone might occur, depending on the water table depth, soiltexture and structure, and seepage.Runoff in irrigated agriculture is mainly related to the intensity of irrigation and rainfall eventsin comparison to the infiltration capacity of the soil. Where the infiltration rate is smaller than theirrigation or rainfall intensity, water will accumulate on the soil surface and run off under aminimum surface slope. Soil degradation, in terms of compaction and crust formation as well ascultivation on steep slopes, promotes surface runoff. Through the physical forces of the runningwater, soil particles become suspended in the water and are transported to open drains, ditches,streams, rivers and lakes. Deposition of suspended sediments may occur downstream whencurrent velocities decrease. Suspended soil particles are harmful to aquatic life as they diminishlight transmission, but also because chemical contaminants may be associated with suspendedsediments (NRCS, 1997). Degradation of drainage water quality as affected by runoff fromagricultural land is especially important in hilly areas and in areas with excess rainfall.In the more arid areas and in flat plains, water flowing through the soil profile and associatedsolute fluxes affected by mineral solubility and adsorption processes are of more importance forthe final quality of the drainage effluent than surface runoff. In some places, weathering of soilparticles might play a major role in the quality of drainage water (such as dissolution of gypsum).However, in general the soil’s ability to adsorb and release through ion exchange and transformchemical elements through microbially-mediated redox reactions plays a more important role. Inthis context, soil characteristics such as water holding capacity, hydraulic conductivity, clay andorganic matter content, soil minerals and soil microbes are important characteristics. Soildegradation in the form of erosion, compaction and loss of biological activity reduces the waterand solute holding capacity of the soils. This increases the mobility of solutes through the soil