Study On The Impacts Of Drought On Water Environment In Majia River Basin

Recent years, EHP (Extreme Hydrology Phenomenon) resulted from climatechange tend to happen frequently, which also lead to series of ecological effects suchas water resources scarcity, water quality deterioration, biodiversity decrease, etc.Many studies have been done as to the influence of EHP on water yield, however,relevant research on water quality are relatively less. From the environment impactpoint of view, drought events are considered to be more dangerous due to theirmedium-term to long-term characteristics and large spatial impacts. On the one hand,during the drought period，projected changes in air temperatures and rainfall couldaffect the physical and biochemical reactions, combined with deteriorations in surfacewater quality, on the other hand, for the decrease in runoff during drought periodshould result in and decrease in pollutant transport, non-point pollutants areaccumulated in earth surface, the changes in pollutant transport is opposite to extremeprecipitation events, especially after a drought period, so drought-rewetting cyclesmay impact water more significantly. Therefore，it is necessary to understand theaction mechanism of droughts and water environment for the frequency and intensityof droughts is increasing under the current climate background, and acquire theleaching rate and pollution rate of non-point pollutants in different drought-rewettingscenario. It has significant important to water pollution control, water resourcesprotection, ecological environment protection and adaption climate change.In this paper, the interaction mechanism between drought and water environmentare figured out with guidance of “nature-society” binary water cycle theory andbackground on hydrology, meteorology, ecology and their inter-disciplines. Besides,the loss amount and leaching rate changes of non-point source pollutants are analyzedunder various conditions of wetting and drying after a lot of work on field observation,simulating lab test, statistical analysis and GIS analysis. Finally, agricultural non-point source pollutants load capacity after single precipitation is evaluated andrelative measures are concluded：(1) Human activities are main sources of environment deterioration while EHPcan make the situation worse. In drought period, mass of non-point source pollutantsare accumulated since the overland runoff is less. Once extreme precipitation happens,the surface pollutants will be washed out into the rivers, which will also cause thewater environment along the river basin to deteriorate rapidly. Therefore, alternationof wetting and drying are especially influential to water environment.(2) Precipitation duration and intensity will both aggravate the nitrate nitrogenand ammonia nitrogen loss in farmland soil when the soil moisture content is certain.Under the same precipitation duration and intensity, the nitrate nitrogen and ammonianitrogen loss in farmland soil will increase as soil moisture decreases and the impactsof precipitation duration and intensity on nitrate nitrogen will be more remarkable.(3) When soil moisture is certain, the nitrate nitrogen and ammonia nitrogenloss in farmland soil tends to increase as precipitation intensity increases; on thecontrary, when precipitation intensity is certain, the low the moisture, the high the loss.Overall, nitrate nitrogen loss depends on drought and precipitation more noticeablythan ammonia nitrogen loss.(4)A negative correlation between leaching efficiency of nitrate nitrogen andammonia nitrogenand soil moisture is created while a positive correlation related toprecipitation intensity is also created. The leaching efficiency of nitrate nitrogen andammonia nitrogenwill be pretty high when precipitation intensity increases and soilmoisture decreases. What’s more, the nitrate nitrogen loss is more easily influenced. Itis found that precipitation intensity also affects the leaching channel of nitrogen. Forexample, when soil moisture is under30%and precipitation intensity is3.5mm/min,nitrate nitrogen is mainly leached by direct surface runoff (pollutants amount reaches76.40%).(5) Tuhai Majia river basin has been frequently influenced by drought in recent50years. In summary, continuous rainy days increases, rainy days decreases, slightlyrainy days decreases noticeably, heavy rainy days don’t have remarkable changes. Alternation of wetting and drying has become the general trend of Horse Cheek riverbasin. The river basin experienced60-day non-rainy days in June,2012. It is reportedthat COD content at every point is measured excessive; TN content at some pointsmet V level of surface water environment quality. NO3-Nand NH4-N contentsbasically met III level. Meanwhile, the leaching efficiency of NO3-N and NH4-Nalong the Horse Cheek river basin was evaluated to be18167kg and795053kgrespectively after a intensive precipitation on June29th.Finally, from the viewpoint of comprehensive measures of water environmentand water resources, taking the combination of prevention and treatment, mainly inprevention and ecology preferential as the core ideas, producing the comprehensiveresponses from the respects of water resource management, non-point sourcepollution controlling, joint controlling of reservoirs and laws and regulations.