| Jinghe is the important water sources of loess plateau, crossing semi-humid and semiarid area in the northwest loess plateau. In recent decades, with the variation of climate and land use caused by temperature rising and vegetation coverage change, the production of runoff in Jinghe significantly continued reducing, that endangered the safety and sustainable development of water resources in loess plateau. Therefore, it was urgent to carry out deep research to know the hydrological processes and factors influencing hydrology and water resources. We selected the Jinghe upstream controlled by the Jingchuan gauge station as research area. Based on terrain, vegetation, soil, meteorology and hydrology observation and investigation data, using statistic analysis and the eco-hydrological model SWIM, the impact of variation of climate and vegetation coverage to hydrological process and amount of water resource was quantified. The result can provide theoretical basis and scientific guiding for water resources allocation, vegetation construction integrated management and sustainable development of arid area, and also provide decision-making basis and reference for making scheme of basin water resources management and land use and vegetation pattern planning under climate change. Specific research main results are as follows:1 Hydrologic situation changesThis paper analyzed the characteristics of climate and runoff of Jinghe river upstream in the past 52 years (1954-2005) and fund that the runoff series showed a significantly downward tendency from 72mm in 1950s to 51.4mm in 2010s, while the precipitation was even and pan evaporation had weak downtrend. The reduciton of runoff was caused by various factors, and the main influencing factors was different in ages. But the impact of human activities strengthened continually, especially compared with period of 1954-1960, the runoff of 1998-2005, when the grain for green and forest were restored and increased, significantly reducted 23.1mm.2. The applicability of the SWIM in Jinghe upstreamSWIM model could accurately simulate the basin potential evapotranspiration, precipitation interception, forest transpiration and farmland evapotranspiration and runoff, and obtained the main parameters in loess and moutains respectively. In Jingchuan station, the R2 and Nash-Sutcliffe coefficient in calibration and validation are 0.51 and 0.55 respectively, with runoff relative error of 1.6% and 0.3%, and both of the trend of runoff peak value of simulated and measured was same. Meanwhile, model application existed the uncertainties and limitations. Less Rainfall stations of rocky mountains directly affected runoff simulation precision, and Single outlet runoff validation could not guarantee the accuracy of inter-subbasin runoff simulated, and only the typical vegetation evapotranspiration or its components had been verified, but some other had not been validated because of study conditions.3. Spatio-temporal distribution of water balance components in Jinghe upstream(1) the annual average precipitation was 547mm in Jinghe upstream in 1997-2003, and the simulated potential evapotranspiration and actual evapotranspiration was for 934mm and 451mm respectively, including soil evaporation for 253mm, vegetation transpiration for 157mm, canopy interception for 40mm. Potential evapotranspiration increased first and reduced later from the west to the east, but evapotranspiration decreased first and increased last with elevation. Evapotranspiration had a great range of 398-512mm with annual big fluctuation, for that of wet year was more than that of drought year 70mm, with more soil evaporation, transpiration, and canopy interception. Evapo- transpiration was mainly occurred from May to August, with the average monthly evapo- transpiration about 67mm·mon-1, in which canopy evapotranspiration rate reached 63%(2) In the whole basin, calculated by simulated value of each basin unit, annual surface runoff was10.7mm, subsurface runoff 31.9mm, and deep percolation 64mm respectively. Sum of the first two was 42.6, and sum of three 106.6mm. In addtion, the components of water yield had great interannual fluctuation. the surface runoff and subsurface runoff had a big range of 3.5-18.1mm and 6-66mm respectively, and deep percolation -8.5-188.1mm. Meanwhile, water yield ability in different area was very distinct, and increased with elevation rising. In loess, surface runoff was 12.5mm, subsurface runoff 2.5mm, and deep percolation 59mm. But in moutains, subsurface runoff was 79.7mm, surface runoff 7.8mm, and deep percolation 72mm.4. The typical vegetation hydrological effect(1) The average annual evapotranspiration for woodland and shrub land was 479-519mm, with 86%-99% of rainfall, except that of the shrub land in the rocky mountains was slightly lower for 429mm. Forest generally had higher interception and transpiration for 86-117mm and 196-258mm respectively, and a low soil evaporation for 138.5-197 mm. Forest water yield is 63-119.5 mm in the Rocky Mountains, which is mostly subsurface runoff with less soil percolation of 4.6-15.5mm, except shrub land with higher soil percolation of 79mm due to thin soil. But without any water yield, the forest in loess area needs water of 8.4-13.8 mm from outside to provide its evapotranspiration. (2) Evapotranspiration of various farmlands was 426-470mm, with the lowest value for mountain crop and highest value for flat crop. Canopy interception of farmland was for 20-41 mm, annual soil evaporation for 231-264mm, and transpiration in loess area and Rocky Mountains with distinct difference for 175-232 mm and 143mm respectively. Water yield of mountain crop among farmlands was the highest for 160mm, with the runoff of 97mm and soil deep percolation of 63mm. But water yield of farmlands in loess area was 44-70mm, with soil deep percolation of 40.5-64mm.(3) Among the grassland types, evapotranspiration of artificial grassland was the biggest for 504mm, with higher canopy interception of 38.5mm, greater transpiration of 210mm, and lower soil evaporation of 256mm. Evapotranspiration of loess natural grassland was lower for 420mm, and that of mountain natural grassland was lowest for 378mm. And both of them had less canopy interception of 13.5mm and 16mm, lower transpiration of 105mm and 48.5mm, and higher soil evaporation of 310mm and 313mm respectively. Water yield of mountains natural grass was highest for 213.5mm than that of loess natural grass for 93.5mm and that of artificial grass for 58mm. But water yield of them was main for soil deep percolation for 180mm, 87 mm and 58mm, with less runoff for 33.5 mm, 7mm and 0 respectively.5. Impact of Land use changes on water balanceIn the loess area or rocky mountains, the amount of evapotranspiration from big to small was followed by forest, farmland and grassland, and the ability of water yield from small to big followed by forest, farmland and grassland. In loess, changing terrace wheat and grassland into the forest, the site evapotranspiration could significantly increased 66mm and 83-87mm, and also water yield significantly reduced 90mm and 101-107mm, in which the runoff reduced 8mm and 5-6.6mm and deep percolation reduced 82mm and 95-102mm. Changing farmland into loess grassland, evapotranspiration also significantly reduced 27mm, but water yield increased 23mm, in which runoff reduced 0.5mm, but deep percolation increased 23mm. In rocky moutain, changing moutain wheat land to forest and shrub, the total evapotranspiration increased 76mm and 44mm, the water yield reduced 85mm and 49mm, respectively. while changing that to moutain grass, the totall evapotranspiration significantly reduced 78-84mm, runoff increased 41.5-53mm, and deep percolation increased 30-45mm.As the forest coverage rate increased 10%, the average total evapotranspiration of whole valley could increase 8mm, mainly for canopy evapotranspiration increased 16mm, but soil evaporation decreased 8mm.The outlet runoff reduced 7.5 mm with forest coverage rate increasing 10% in rocky mountain, and decreased 4.5mm with that in loess area. According to the slope grade, 12 vegetation restoration scenarios were designed and simulated. Result showed afforestation and planting grass was very necessary for soil conservation in Jinghe upstream. the whole afforestation or planting grass would seriously influence basin water management. Therefore, water resources allocation was need to consider the amount of water in valley. For example, the scenario3 and scenario10, that planted grass in middle slope position with larger slope, and trees in downhill with smaller slope, may make a reasonable use of water resources in Jinghe upstream.6. Impact of climate change to water balance componentIn Jinghe upstream, annual runoff, evapotranspiration and its components linearly increased or reduced with precipitation increase or reduction. Among them, the impact of increasing precipitation to runoff was more significant than that of reducing precipitation, and impact of increasing precipitation to evapotranspiration was less significant than that of reducing precipitation. Runoff transformed from increasing precipitation with proportion of 63% was significantly larger than transformed evapotranspiration with proportion of 23%. And both of them transformed from reducing precipitation had little difference, with proportion of 42.5% and 31%, respectively.As temperature rise or lower, annual runoff showed reducing or increasing trend, and evapotranspiration and its components showed increasing or reducing trend. The influence of temperature rising to annual runoff was obviously less than that of temperature lowering. And the influence of temperature rising to evapotranspiration was less than that of temperature lowering. Among components of annual evapotranspiration, response of soil evaporation to temperature change was larger than that of canopy evapotranspiration, and the influence of temperature rising to them was less than that of temperature lowering. |
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