Environmental Problems, Their Causes, and Sustainability 1

OvergrazingDeforestationErosionCausesSalinizationSoil compactionNatural climatechangeConsequencesWorsening droughtFamineEconomic lossesLower livingstandardsEnvironmentalrefugeesFigure 14-10 Causes and consequences of desertification.sorbed into the soil evaporates, leaving behind a thincrust of dissolved salts (such as sodium chloride) inthe topsoil.Repeated annual applications of irrigation waterlead to the gradual accumulation of salts in the uppersoil layers. This accumulation of salts is called salinization(Figure 14-11). It stunts crop growth, lowerscrop yields, and eventually kills plants and ruins theland (see figure on p. ix, left).EvaporationEvaporation TranspirationEvaporationAccording to a 1995 study, severe salinization hasreduced yields on about a fifth of the world’s irrigatedcropland, and almost another third has been moderatelysalinized. The most severe salinization occurs inAsia, especially in China, India, and Pakistan.Salinization affects almost one-fourth of irrigatedcropland in the United States. But the proportion ismuch higher in some heavily irrigated western states.We know how to prevent and deal with soilsalinization, as summarized in Figure 14-12. But someof these remedies are expensive.Another problem with irrigation is waterlogging(Figure 14-11). Farmers often apply large amounts ofirrigation water to leach salts deeper into the soil. Butwithout adequate drainage, water accumulates undergroundand gradually raises the water table. SalineSolutionsSoil SalinizationWaterloggingPreventionCleanupLess permeableclay layerReduce irrigationFlushing soil(expensive andwastes water)Salinization1. Irrigation water containssmall amounts ofdissolved salts.2.Evaporation and transpirationleave saltsbehind.Waterlogging1.Precipitation andirrigation waterpercolate downward.2. Water table rises.Switch to salttolerantcrops(such as barley,cotton, sugarbeet)Not growing cropsfor 2–5 yearsInstallingundergrounddrainage systems(expensive)3. Salt builds up in soil.Figure 14-11 Natural capital degradation: salinization andwaterlogging of soil on irrigated land without adequatedrainage can decrease crop yields.Figure 14-12 Solutions: methods for preventing and cleaningup soil salinization.http://biology.brookscole.com/miller14283