| Land cover change and climate change has been firmly believed as the key impact of watercycle and water balance in forest watershed with large scale.The Zagunao Riveristhe upperstream of Minjiang River which is the largest tributary of Yangtze River. The watershed lies onthe transition zone of Qinghai-Tibet Plateau to Sichuan basin where climate change impactsignificantly on local ecosystem.The vegetation in Zagunao watershed is relatively simple,which has almost been changed by massive forest harvesting and forest recovery. As a hypothesis,we assumed that climate change and massive forest variation has influence regional water cycleand water balance andquantified and discussed the whole relation and process in this paper basedon both statistic and modeling way in both temporal and spatial perspective. The main resultsand conclusions are as follows:(1). We analyzed the hydrological regime in Zagunao watershed during1958-2005basedon the observed data of6weather stations and the hydrology station on the outlet of Zagunaowatershed. The results showed that, the annual precipitation, and annual runoff and runoff in wetseason had no significant tendency during1985-2005(P>0.05, tau=0.05,0.126,0.067), whilerunoff in dry season had a significant decrease (P<0.05, tau=0.23).Beaks have been detectedwith the double mass curve of runoff and precipitation in Zagunao river during1958-2005basedon Mann-Kendall test. Annual runoff significantly increased after1969(P<0.05, tau=0.13) andvariated a lot after1991respectively. Runoff in wet season significantly increased after1965(P<0.05, tau=0.084) and decreased after1998respectively (P<0.05, tau=-1.361). Furthermore,the breaking time was quite coincide with forest variation period.(2). The best substream quantity in Zagunao watershed was27(R2=0.72and E=0.33). Thenthe parameters of vegetation and soil type was determined based on investigation and earlypapers. SWAT model performed well in Zagunao watershed after complex parameterization and calibration: for monthly flow, daily flow, and ET, the R2was0.89,0.78and0.90respectively andE was0.81,0.51and0.80respectively.(3). The percentage of ET and water yield to precipitation was39.24%and60.26%respectively, transpiration/ET, interception/ET and evaporation/ET was41.78%，15.75%and42.47%respectively, and surface runoff/runoff, lateral flow/runoff and base flow/runoff was33.30%，54.77%and11.32%respectively. Water yield in forest was higher than alpine meadow,crop, and shrub significantly (P<0.05,166.82mm，126.15mm，155.03mm), while ET was lowerthan alpine meadow, crop, and shrub significantly (P<0.05,128.61mm，88.60mm，103.21mm).ET of plantation was higher than coniferous forest and mixed forest (87.13mm，11mm), whilethe sueface runoff of plantation was also higher than coniferous forest, mixed forest andbroadleaved forest (51.76mm，37.09mm and68.09mm) which indicated that coniferous forestwas better in conserving water, while plantation had both disadvantage in higher ET and surfacerunoff.(4). The runoff increased with clear cutting area in our clear cutting scenario (5%-40%)with spatial heterogeneity: ET increased1.73mm once clearcuting area widen by5%whenaltitude was blow3221m and decreased2.86above3221m. After harvesting, Water yieldefficiency decreasedas the stand age increaseduring the tenth to thefortieth year. Meanwhile, theET efficiency,transpiration efficiencyincreased by2.67%and1.56%, and the water yieldefficiency and evaporation efficiency decreased by1.43%and1.31%every ten year as forestgrow.(5). Precipitation had a stronger influnce than temperature on water cycle in Zagunaowatershed. Water yield increased by7.9%,168.7%, and ET increased by0.8%once precipitationincreased by10%,20%,30%. On the contrast, water yield decreased by4.2%, and ET decreasedby1.3%once precipitation decreased by10%. Meawhile, Increasing of temperature had a largerinflunce on water yield than decreasing while few difference was there on the impact to ET. |
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