The Impacts Of Typical Measures Of Soil And Water Conservation On Ecological Basic Flow Recharged From Groundwater

The decreasing of water and sediment discharge in rivers and the shortage of ecological basic flow had a serious impact on the social, economic and ecological security, so it had caused wide attention. However, the impact mechanism of the soil and water conservation measures on the process of river runoff recharged from groundwater is not clear yet. The Wei River, which has a serious water shortage and a high sediment concentration, was selected as the research watershed. This study was based on the statistical analysis of measured data and the combination of simulation tests indoor and mathematical models. A simulation testing system which can simulate the transformation of water between precipitation, runoff, soil water and underground water was designed, and the impacts of soil, slope, rainfall intensity and measures on process of the transformation of precipitation and groundwater flow was studied. Then, a precipitation and runoff SWAT model using the measured data of the watershed was constructed to analyze the effect of soil and water conservation measures on the recharge of river runoff especially on the ecological basic flow from underground water. The main conclusions were as follows:(1) The test system design. In order to study the effect of soil and water conservation measures on groundwater recharge, on the basis of generalizing the transformation model of rainfall runoff to surface water and groundwater in Wei River where has strong groundwater runoff recharge intensity, a simulation test system was designed. This system was a rainfall-runoff-erosion-groundwater-streamflow indoor simulation system, which included rain system natural system, observation system etc. The system simulated rainfall with automatic simulation device of under sprinkler. The different underlying surfaces were designed to simulate different soil and water conservation measures. The different mechanical composition soil was used to simulate the migration of soil water and groundwater. The piezometric tubes were installed in left side to observe changes of groundwater level. The Neutron Probe was used to measure soil water. And the conventional methods were used to measure velocity and flow of surface water and groundwater. The results showed that this system could well simulate the transformation process of rainfall, surface runoff, soil water, groundwater and river runoff under slope condition.(2) The simulation results showed that the typical measures of soil and water conservation could promote the precipitation transform into groundwater flow. Based on the screening of the typical soil and water conservation measures, the artificial rainfall simulation experiments were carried out. The results showed that compared with the bare slope, the grass and straw mulching measures could reduce surface runoff, increase soil moisture and promote the rainfall recharge groundwater significantly. Under the 3o slope, 45~120mm/h rainfall intensity conditions, the amount of groundwater recharge increased by 1.96 and 2.05 times through measures of lawn and straw mulching, compared to bare slope condition. In general, the rainfall intensity, soil particle size and gradient of slope had significant effects on the surface and underground flow from rainfall under the condition of bare slope. And the measures of grass and straw mulching weakened these factors influence on the relationship of rainfall runoff, which was good at the transformation of surface water to the groundwater.(3) In order to study the impact of different soil and water conservation measures and different rainfall intensities on groundwater recharging in river runoff better, the MODFLOW mathematical model was chosen. Results of verification in groundwater balance, levels, runoff and flow field all showed all error indicators were within the allowable range. MODFLOW could be used in this study. Meanwhile the calibration results showed recharge rate(α), specific yield of three soil layers(Sy) were the most sensitive parameters for the change of underlying surface. Under the same measures and rainfall intensities, different slope conditions, the amount of groundwater recharge decreased with the slope increases. Under the same slope and rainfall intensity, different measures conditions, the amounts of groundwater recharge was in sequence of straw mulching> grass land> bare slope. Under the same measures and slope, different rainfall intensities conditions, the amount of groundwater recharging runoff decreased with the rainfall intensity increasing. Under the 20 o slope and 120 mm rainfall conditions, when the rainfall intensities of bare slope, grass land and straw mulching respectively were larger than 105mm/h, 255mm/h and 285mm/h, the rainfall was difficult to recharge groundwater. Under the 150mm/h rainfall intensity and 120 mm rainfall conditions, when the slopes of bare slope, grass land and straw mulching respectively were larger than 15 o, 15 o and 40 o, the rainfall was difficult to recharge groundwater.(4) Based on the soil conservation measures in favor of groundwater recharge analysis in the slope, the measured data of Wei River basin showed that with the improvement of comprehensive harness of soil and water conservation, the effect of groundwater recharged river ecological basic flow was obvious. The measured data of rainfall, soil moisture and runoff was analyzed by using wavelet analysis, Mann-Kendall test, double cumulative curve and digital filter and other statistical methods. The results showed that the base flow index was positively correlated with the water storage capacity of basin in the dry season. Before the 80 s, with the destruction of the ecological vegetation, the base flow index continues to reduce in non-flood season; since the 80 s, with the implementation of the comprehensive management of watershed, the base flow index showed a trend of increase in dry season. With the rainfall reducing first and then increasing, the soil moisture of farmland continued increasing in flood season. The analysis results showed that soil and water conservation measures could delay the flood and add the drought season runoff, which was good at the ecological basic flow protection in the dry season.(5) In order to investigate the influence of typical soil and water conservation measures on the recharge of groundwater to river, the effects of forest and grass measures in Wei River basin on the variation of runoff recharged by groundwater were analyzed through SWAT model. Using the information data of hydrology, meteorology, soil and land use of the basin, the SWAT model for the middle reaches of the Wei River basin was built. The monthly runoff data of Linjiacun, Weijiabu and Xianyang hydrological stations duiring 1960-1969 and 1970-1979 were adopted respectively for model calibration and validation. Nash coefficient(NS) and the square of the correlation coefficient(r2) ranges were 0.82~0.84 and 0.79~0.86 respectively in calibration period. They were 0.70~0.76 and 0.74~0.79 respectively in validation period. The model could be used in this study. On this basis, four scenarios that present situation, farmland was converted into woodland or grassland respectively by 10%、20% and 40% were simulated in wet year, normal year and dry year. The results showed that for the normal year in non-flood season, in Linjiacun hydrologic section, 0.034m3/s of ecological basic flow increased with every 1% area of woodland increasing above the section, which improved the guarantee rate of recent ecological basic flow about 1.30%. In Weijiabu hydrologic section, 0.082m3/s of ecological basic flow in non-flood season increased with every 1% area of woodland increasing above the section, which improved the guarantee rate of recent ecological basic flow about 2.06%. In Xianyang hydrologic section, 0.096m3/s of ecological base flow in non-flood season increased with every 1% area of woodland increasing above the section, which improved the guarantee rate of recent ecological basic flow about 1.81%. And for normal year, every 1% area of grassland increasing above the hydrologic sections of Linjiacun, Weijiabu, Xianyang, the guarantee rates of recent ecological basic flow increase 1.15%, 1.71% and 1.28% respectively.