Experimental Study On The Shallow Groundwater Dynamics And Recharge Process In The Loess Plateau

In order to investigate the recharge process and mechanism of shallow groundwater in the small watershed of the Loess Plateau,this paper uses the Liudaogou small watershed?water-wind erosion crisscross region?in Shenmu,the Kangpinggou small watershed?Loess Plateau hilly area?in Yan'an and Wangdonggou small watershed?Loess Plateau tableland area?in Changwu as a case study.Soil saturated hydraulic conductivity?K_s?of sloping field was measured in situ using the three single-ring infiltrometer methods,and the effect of slope gradient on K_s by three single-ring infiltrometer methods was analyzed.Based on the measured datas of the shallow groundwater level and base flow in check dams,the characteristics of spatio-temporal changes of rainfall,base flow and groundwater were analyzed.The relationship between rainfall,base flow and groundwater was discussed,and the response of base flow and groundwater to rainfall replenishment was studied.The recharge process and approach of shallow groundwater in the small watershed of the Loess Plateau was explored.Here are the main results.?1?Three single-ring infiltrometer methods?i.e.,two-ponding depth,one-ponding depth and BEST methods?were used to measure the sloping soil saturated hydraulic conductivity of three different soil types such as sandy soil,Loessial soil,and Lou soil.The results showed that all three methods obeyed the same order on flat soil surface for K_s of different soils as sandy soil>Loessial soil>Lou soil,but single-ring BEST infiltrometer method was significantly higher than single-ring two-ponding depth infiltrometer method and single-ring one-ponding depth infiltrometer method?P<0.05?.K_s measured by single-ring two-ponding depth infiltrometer method increased with the increase of slope gradient,while the single-ring BEST infiltrometer method had the opposite result.As the texture of the soil becomes fine,the degree of influence of the slope gredient?the slope of the regression line?tends to decrease.K_s measured by single-ring one-ponding depth?10 cm?infiltrometer method is completely consistent with single-ring two-ponding depth infiltrometer method.After the soil hydraulic parameters are determined,it can replace single-ring two-ponding depth infiltrometer method.When the slope<10°,there is no significant difference between the K_s measured by the three methods on the flat land.When the slope>10°,the difference is significant?P<0.05?.?2?In the water-wind erosion crisscross region of the Loess Plateau,rainfall is an important recharge source for shallow groundwater and base flow.The shallow groundwater stored in the check dams shows a regular law of alternate peaks and valleys with seasons.The thickness of soil layer is between 4.09 m and 10.35 m in check dam A,and the thickness of soil layer is between 4.52 m and 6.72 m in check dam B.The shallow groundwater level began to decline significantly in May 2017.From June 1st to August 25th,A₈₀ and B₅₀ dried up,and the A₃₄₀ and A₅₁₀ groundwater depths were the deepest on August 29th and July 22th,which were 7.77 m and 7.85 m respectively.Both B₁₅₀ and B₂₈₀ have the deepest groundwater depths on August 6th,which are 4.51 m and 4.54 m respectively.The water level began to rise in September.The depth of shallow groundwater table became shallower in the fall and winter period.The groundwater depths of A₈₀,A₃₄₀ and A₅₁₀ were shallowest on September 5th,October 29th and December 25th,which were 2.34 m,7.10 m and 6.86 m.The groundwater depths of B₅₀,B₁₅₀ and B₂₈₀80 were the shallowest on December 1st,September 25th and December 12th,which were 3.08 m,3.33 m and 3.76 m,respectively.Rainfall could rechange to shallow groundwater and shallow groundwater is supplied to the base flow of the channel by the outflow of spring water formed at the junction of loess and bedrock.There are mainly two ways to supplement the shallow groundwater in the check dam system:1)When the groundwater depth of the check dam is less than 4 m,more than 30.0 mm rainfall can enter groundwater vertically.2)When the groundwater depth of the head in check dam is less than4 m and the groundwater depth of the middle and tail in check dam is greater than 4 m,the runfall is collected from small watershed and then fed to the groundwater through the vertical infiltration of the gully head,and then moved from the head to the tail of the check dam by lateral seepage.The year-to-year change in the base flow has the law of alternation of peaks and valleys.In March,the temperature gradually rises,and the permafrost begins to melt.The base flow also gradually increases and reaches a peak value on May 13th with a value of471.2 m³·d^-1.Afterwards,the vegetation in the small watershed grew vigorously,the water consumption and transpiration of the plant were strong,and the base flow gradually decreased.It decreased to the lowest value on the 13th of July,with a value of 37.0 m³·d^-1.With the advent of the rainy season,the base flow increased continuously and reached a sub-peak value on September 3rd,with a value of 245.9 m³·d^-1.There is a certain lag period between the response of shallow groundwater and base flow to rainfall.The lag time of the base flow to rainfall is about 4-16 days.When direct vertical infiltration is used to recharge shallow groundwater vertically,the lag time of shallow groundwater to rainfall is about 12-54 days.When rainfall is fed through the gully head sinker and seepage to supply shallow groundwater,the lag time is about 72-93 days.?3?In the Loess Plateau hilly area,rainfall can also supply to shallow groundwater,but there is a certain lag time.The thickness of soil layer in check dam D is about 4.55⁶.55 m from the head to tail of the check dam.The thickness of soil layer is an important factor affecting the rainfall response.From the head to tail of the check dam,with the thickening of the soil layer,the sensitivity of the shallow groundwater level response gradually decreases.The shallow groundwater stored in the check dams shows a regular law of alternate peaks and valleys with seasons.The shallow groundwater level began to decline at the end of April each year.D₃₀,D₁₁₅ and D₂₀₀ had the deepest groundwater depths on August 17th,July 24th,and July 22th,which were 1.42 m,3.69 m,and 4.94 m,respectively.During the fall and winter period,the depth of shallow groundwater table became shallower.D₃₀,D₁₁₅ and D₂₀₀ had the shallowest groundwater depths on November 5th,October 19th,and November 27th,which were 0.21 m,3.20 m and 4.30 m,respectively.Rainfall can be supplied to shallow groundwater,and shallow groundwater can be supplied to the base flow of the channel by a spring outflow formed at the junction of loess and bedrock.In autumn and winter,the base flow is large,and the base flow in summer is the smallest.After May,the vegetation in the small watershed entered the vigorous growth period,and the base flow decreased.The minimum base flow was 1.0 m³·d^-1 on July 8th.In August,with the onset of the rainy season,the base flow was continuously increased and increased.The maximum base flow was on October 26th,with a value of 7.8 m³·d^-1.There is a certain lag period in the response of shallow groundwater to rainfall.When rainfall is fed through the gully head sinker and seepage to supply shallow groundwater,the response time delay is about 31-49 days.?4?In Loess Plateau tableland area,the precipitation in the year showed a trend of“less-more-less”,and the solar radiation and air temperature also showed“small-large-small”trends,but the base flow was“large-small-large”change trend.The base flow was the smallest on July 17th with an average of 9.1 m³·d^-1 and which was the largest in November29th,with an average of 13.4 m³·d^-1.There was a significant linear correlation between the amount of rainfall and the base flow?P<0.01?.As the amount of rainfall increased and the amount of change in base flow increased,it was suggested that rainfall may be infiltrated into shallow groundwater and discharged in the form of base flow to channel.?5?Shallow groundwater of check dam system in water-wind erosion crisscross region and Loess Plateau hilly area changes with the seasons,and there is a certain lag period for the response of shallow groundwater to rainfall.From May to September,during the growth cycle of vegetation,the groundwater level drops first and then rises.Groundwater depth and rainfall are closely related to the replenishment of shallow groundwater.Rainfall greater than 30.0mm,in water-wind erosion crisscross region,check dams with groundwater depths greater than 4 m and less than 4 m have two different recharge routes such as seepage lateral infiltration and direct vertical infiltration,but in Loess Plateau hilly area,a check dam with a groundwater depth of more than 4 m is used as the vertical infiltration of the gully head and lateral seepage route.The response lag time of shallow groundwater in the water-wind erosion crisscross region to rainfall is greater than that in the Loess Plateau hilly area,about 42 days.The base flow in the water-wind erosion crisscross region,Loess Plateau hilly area and Loess Plateau tableland area is small in spring and summer,and is relatively large in autumn and winter.There is also a daily variation law of peak-valley alternation in the base flow,and the water temperature and the flow rate show the opposite trend.Taking the period from July 30th to August 3th,2017 as an example,the time for peak and minimum flow in the three regions is different.The base flow in water-wind erosion crisscross region reached the peak of the current day at 6 am and the value was 7.93 m³·h^-1.The flow was the lowest at 16 pm,with a value of 4.54 m³·h^-1.The base flow in the loess hilly area reached the peak of the current day was about 3 am and the value was 0.09 m³·h^-1,and the base flow was the lowest at 14 pm,with a value of 0.06 m³·h^-1.The base flow in the Loess Plateau tableland area reached the peak at the time of day at 16 am,with a value of 0.42 m³·h^-1,the minimum flow at 0 o'clock,with a value of 0.37 m³·h^-1.Rainfall,loess thickness and soil texture are closely related to the daily variation of base flow.