Effects of high intensive vegetation restoration on groundwater recharge in ecologically fragile mining area

Taking the transmission processes of the precipitation,soil water and groundwater as the research object,the methods of remote sensing,in-situ test and numerical simulation were used to study the effects of high intensive vegetation restoration on groundwater recharge in arid mining area combined with the observation data in bare soil area and vegetation covered area. The results showed that both the normalized difference vegetation index and the net primary productivity of vegetation demonstrated a continuous increase in the mining area which indicated a continuous decrease of the low coverage area and a continuous increase of the medium and high coverage area. The largest total soil water potential was at the depth of 20 cm in the bare soil area which manifested that the soil water moved upward and downward from the depth of 20 cm,therefore it was easy to receive the recharge of precipitation. However,the total soil water potential in the vegetation cover area was much lower than in the bare soil area and there was a low total soil water potential zone in the root distribution area which manifested that the soil water moved from the upside,downside and lateral to the root distribution area,therefore it was difficult to receive the recharge of precipitation. The groundwater level showed a slow rising trend with a low intensity infiltration in bare soil area. However, the groundwater level showed an obvious trend of continuous decline with a high intensity of evaporation in the vegetation covered area. Owing to the influence of vegetation transpiration,the vertical water exchange capacity at the groundwater level increased from -0. 035 cm / d ( bare soil area) to 0. 48 cm / d ( vegetation covered area),which changed the direction and amount of water transmission. With the increase of leaf area index (LAI),the water consumption of vegetation tended to increase. Therefore,the downward soil water flux gradually decreased and the groundwater recharge decreased which showed that with the increase of leaf area index,the precipitation infiltration coefficient gradually decreased. For instance,the numerical simulation results showed that the infiltration coefficient of wind-blown sand was 0. 54 under bare soil condition (LAI = 0),and it decreased from 0. 54 to 0. 198 with the increase of leaf area index from 0 to 3. 5,while this ratio was near 63. 3% which had a great influences on the groundwater recharge in the Yushen mining area. It is better to choose the vegetation type with small water consumption and an appropriate vegetation coverage for the engineering practice of vegetation restoration in Yushen mining area which can avoid the phenomenon that the precipitation infiltration coefficient decreases greatly due to the excessive regional ecological water consumption. The results of this study have a scientific guidance for enriching the harmonious development of vegetation restoration and groundwater resources protection in the western ecologically fragile mining area of China.