Using Visual MODFLOW Model to Assess the Efficiency of Subsurface Barrier Wall for Groundwater Flow Regulation and Reduction of Saline Intrution

Barrier walls effectively store water,regulate underground flows,improve exploitable reserves and prevent saltwater intrusion.The effectiveness of the underground barrier wall depends not only on the hydrogeological structure,the technical parameters of the wall but also on the layout scheme of the exploitation well system.The results showed that in natural conditions,the ground water level upstream of the barrier wall rose in the presence of a barrier wall.In wells located downstream of high barrier walls,the water level decreased.The amount of underground current flowing into the sea decreased,the annual average value of the whole region decreased was 316 m3/day and night.In presence of a wall,both the water level and the amount of evaporation increased.The average increase in evaporation volume during the calculation period of ten thousand days with walls was 4.114 m3/d.So in presence of a wall,the amount of water that can be exploited increases by the total amount of evaporation plus the decrease in discharge to the sea and is equal to 4,424 m3/d.In the exploitation condition,if the water level in the presence of wall is kept as low as in the absence of wall,the exploitation flow will increase to about 4,400 m3/day and night.From the calculated water level values when there is a wall and without a wall,we can see that if the exploitation flow in presence of a wall and in the absence of wall is the same,the water level drop at the calculated observation wells upstream of the wall will decrease from 0.21 m to 3.97 m.The condition of effective exploitation of the wall depends on the mining scheme.The exploitation scheme is reasonable,the exploitation flow of the wells does not exceed the allowable flow so as not to cause the drying of the aquifer at the location of the well.The upstream area of the wall reflects quite clearly as the Total dissolved solids content in observation wells upstream of the wall at the end of the calculation time is significantly reduced compared to that without the wall,ranging from 69 mg/L to 5,629 mg/L.In the presence of a wall,the water level of observation wells upstream of the wall is higher than that of without a wall from 0.10 m to 0.74 m.

Time lag characteristics of sap flow in seed-maize and their implications for modeling transpiration in an arid region of Northwest China

Plant capacity for water storage leads to time lags between basal stem sap flow and transpiration in various woody plants. Internal water storage depends on the sizes of woody plants. However, the changes and its influencing factors in time lags of basal stem flow during the development of herbaceous plants including crops remain unclear. A field experiment was conducted in an arid region of Northwest China to examine the time lag characteristics of sap flow in seed-maize and to calibrate the transpiration modeling. Cross-correlation analysis was used to estimate the time lags between stem sap flow and meteorological driving factors including solar radiation（R_s） and vapor pressure deficit of the air（VPD_（air））. Results indicate that the changes in seed-maize stem sap flow consistently lagged behind the changes in R_s and preceded the changes in VPD_（air） both on hourly and daily scales, suggesting that light-mediated stomatal closures drove sap flow responses. The time lag in the maize＇s sap flow differed significantly during different growth stages and the difference was potentially due to developmental changes in capacitance tissue and/or xylem during ontogenesis. The time lags between stem sap flow and R_s in both female plants and male plants corresponded to plant use of stored water and were independent of total plant water use. Time lags of sap flow were always longer in male plants than in female plants. Theoretically, dry soil may decrease the speed by which sap flow adjusts ahead of shifts in VPD_（air） in comparison with wet soil and also increase the speed by which sap flow adjusts to R_s. However, sap flow lags that were associated with R_s before irrigation and after irrigation in female plants did not shift. Time series analysis method provided better results for simulating seed-maize sap flow with advantages of allowing for fewer variables to be included. This approach would be helpful in improving the accuracy of estimation for canopy transpiration and conductance using meteorological measurements.

Engineering Strategies on Flood Control in Middle Reach of Yangtze River, China

Flood disaster has been a serious hidden danger since the ancient time. The essential cause for the fact that floods have not been eliminated for hundreds of years is that time honored strategies do not suit the cases of flood prevention. In the view of geological environmental analyses of flood formation and from the synthesis of experiences gained in flood control in the past hundreds of years, sluggish draining of flood, silt sedimentation in channel and building levee blindly constitute the main cause of intractable flood for a long time in the middle reach of the Yangtze River. Draining away silt and water is the only way to stamping out flood disaster. Opening up artificial waterways for flood diversion, draining away the silt of channel into the polders, and storing the flood water are important engineering measures for the flood control and damage reduction.