Search

【Objective】 Subsurface drains are commonly used in drainage system. This paper investigates the effect of their spacing and depth on water movement and salt transport in field soil under flood irrigation in Hetao Irrigation District. 【Method】 The study was based on data measured from a field, which were used to calibrate the HYDRAS-2D model. The validated model was then used to predict spatiotemporal changes in water flow and salt transport when the drain depth was 1.5m by varying the drain spacing from 5 to 100 m, and when the drain spacing was 25 m by varying the drain depth from 0.6 to 2.5 m. 【Result】 ①The model is accurate for simulating both water flow and salt transport, with the coefficient of determination R2 for soil profile moisture and salinity being 0.81 and 0.71, the RMSE error being 0.038 and 0.026, and the Nash coefficient NSE being 0.93 and 0.86, respectively. ②The cumulative displacement and salt discharge increased with the decrease in drain spacing and the increase in drain depth. When the drain depth was 1.5 m, decreasing drain spacing from 100m to 5m increased total drainage by 4.96 times and increased the salt discharge by 5.06 times. When drain spacing was 25 m, increasing drain depth from 0.6 m to 2.5 m increased the total drainage by 64.24 times, and the total salt discharge by 60.08 times. ③The water content in the soil profile decreased with the decrease in drain spacing and drain depth during the drainage period, and the decrease decreased as time elapsed. The salt content in the soil profile decreased with the decrease in drain spacing and depth in non-drainage period, and the decrease increased as time elapsed. ④Horizontally, salt content in the soil increased with the increase in the distance from the drain in non-drainage period, and the increase decreased with an decrease in drain spacing and an increase in drain depth. When drain depth was 1.5 m, the difference in soil salinity between the drain spacing 5 m and 100 m increased from 26.67% in the region proximal to the drains to 39.88% in the middle of the drains. When the drain spacing was 25 m, the difference in soil salinity between the drain depth 0.6 m and 2.5 m increased from 30.56% in the region adjacent to the drain to 50.39% in the middle of the drains. Vertically, soil water content varied in a “sickle” shape with the increase in soil depth. The initial drainage of salt increased with the increase in drain depth, while in non-drainage period it decreased with the increase in drain depth. 【Conclusion】 Reducing drain spacing or increasing drain depth can improve the drainage and salt discharge, thereby reducing water storage in soil profile during drainage period and soil salt content during non-drainage period. Our results suggested that the optimal drain spacing was 15~50 m and the drain depth was 1.0~2.0 m for Hetao irrigation district.

【Objective】 Subsurface tile is a conventional drainage technology to prevent groundwater table from exceeding critical depths and causing waterlogging and soil salinization. In this paper, the effect of buried depth and exit control of the tile on drainage and groundwater depth was examined through field experiment. 【Method】 The experiment was conducted in the Lixin Drainage Test Area in Bozhou City, Anhui province. The tile length was 200 m and the space between adjacent tiles was 30 m. The exit of each tile was connected to a well with its position either kept at a constant elevation (CWTR) or dynamically controlled (DWTR). Drainage from each tile was measured using an electronic water meter, and change in groundwater depth was measured from a borehole drilled in the middle of the tiles. Groundwater flow in the drainage system was simulated using the DRAINMOD model. 【Result】 Shallow-burying the tile and CWTR control of the tile exit had similar effect in controlling groundwater depth and reducing drainage. The DWTR and CWTR were both able to reduce the groundwater table dropping and drainage in the early and late crop growth stages. In the middle growth stage, the groundwater depth under DWTR was significantly lower than that under CWTR, and the daily drainage from the former was significantly higher than that from the latter. For the conventional tile layout, the average groundwater depth under DWTR in a wet year was 13.9 cm lower than that under CWTR, and the drainage of the former was 2.1 times as that of the latter. Changes in the elevation of the tile exit in DWTR altered groundwater flow, and it took 7 to 12 days, depending on the tile space, for groundwater flow to reach a new steady state. 【Conclusion】 Dynamic control of the elevation of the tile exit in subsurface drainage systems can be an effective means to regulate groundwater flow, preventing waterlogging during flood seasons and ensuring sufficient soil moisture during dry seasons.

Flood stress on crops severely constrains food production. From 2011 to 2018, a plot test was conducted to investigate the effects of flooding duration and growth stage on soybean plant height, the number of solid pods, 100-grain weight, yield, and dry matter mass, and their interannual variation. The results showed that the soybean indicators were significantly influenced by the year, flooding duration and growth stage, and their interaction. Under the same flooding duration and growth stage, the smallest plant height, number of solid pods, 100-grain weight, and dry matter mass were observed in 2016; and the largest plant height, number of solid pods, yield, and dry matter mass were observed in 2011. The soybean critical flooding duration for the number of solid pods, yield, and dry matter mass was 3 days, and that for 100-grain weight was 6 days. The flooding duration had no significant effect on plant height. The flooding-sensitive growth stage for soybean plant height and dry matter mass was the seedling stage, and that for the number of solid pods, 100-grain weight, and yield was the flowering-podding stage. When investigating the effects of flooding stress on soybeans, the impacts of interannual variation such as high temperatures and drought on soybean growth and yield should be integrated. [ABSTRACT FROM AUTHOR]

Drainage engineering technology combined with ditch and pond was proposed and its design method were developed to meet the challenge of increasing design discharge that had been caused by the increase of extreme rainfall events and variations of basin underlying surfaces. Considering drainage patterns and data shortage at small ungauged watershed, the design method consisted of the determinations of drainage criteria and drainage hydrograph, and the design of pond and weir sizes. Design storm amount was determined by the statistical analysis of long-term daily precipitation and local design return period, and the design storm duration, usually 24 h, was discretized based on storm temporal distribution (hourly) in local hydrology manual. Given that drainage hydrograph can be approximated by a triangular hydrograph at every discrete time interval, drainage hydrograph was synthetized each triangular hydrograph calculated with the improved SCS (soil conservation service-curve number) model and concentration equations every discrete time interval. Pond and weir sizes were estimated by the trial and error solution of the stage-storage function, weir equation and continuity equation, as the pond outflow rate was close to design drainage rate of the receiving ditch downstream. In the lowland of the northern Huaihe River Plain, the pond of 800 m² was designed for combining with field ditch and installed with the weir of 0.6 m wide and weir coefficient of 1.28, which would be used to drain the runoff from the area of 0.06 km². The pond of 26000 m² was designed for combining with collector ditch and installed with the weir of 2.0 m wide and weir coefficient of 1.28, which would be used to drain the runoff from the area of 1.36 km². The drainage engineering combined with the ditch and pond extended the time of concentration of 0.5 h for field ditch district and1 h for collector ditch district, respectively, under the return periods of 24 h storm of 3-year, 5-year and 10-year. The drainage engineering combined with the ditch and pond reduced peak storm flows of more than 25% under the return periods of 24 h storm of 3-year, 5-year and 10-year. Combined with the pond weir of 0.6 m wide and 0.7 or 0.85 m high, the drainage criteria of the field ditch could be increased from the return periods of 3-year or 5-year to the return periods of 5-year or 10-year, respectively. Combined with the pond weir of 2.0 m wide and 1.8 or 2.3 m high, the drainage criteria of the collector ditch could be increased from the return periods of 3-year or 5-year to the return periods of 5-year or 10-year, respectively. Drainage engineering technology combined with ditch and ponds could be an option to mitigate the flood damage of the drainage district and downstream. However, drainage engineering technology combined with ditch and ponds should be assessed for different weather and soil conditions, and the design parameters of ponds and weir should be evaluated to adapt different ditches and weather conditions. The effects of drought, flood and environment should be considered as the combined drainage engineering technology is used to control flood. [ABSTRACT FROM AUTHOR]

The accurate simulation or prediction of precipitation runoff has been considered as one of the most important bases for resource management and environmental quality assessment of water and soil. The soil conservation service-curve number (SCS) model, one of the most popular runoff prediction models, cannot effectively determine the effect of the antecedent runoff condition (ARC) on runoff amount, which limits the accuracy of the model's runoff prediction. Assuming that antecedent daily precipitation depleted by evapotranspiration and seepage was linear with watershed water storage amount, the new ARC was established based on the recurrence relation of daily rainfall amount and watershed maximum rainfall storage amount. The SCS model was improved by correlating the initial abstraction with the new parameters of the potential initial abstraction and effective rainfall influence coefficient. The potential initial abstraction determines the maximum watershed rainfall storage amount prior to runoff and the threshold of daily effective rainfall amount, and the effective rainfall influence coefficient describes the dynamic depletion of antecedent daily effective rainfall amount induced by evapotranspiration and seepage. To reduce the number of unknown parameters, the relationship between the potential initial abstraction and the curve number was established under the condition that there was no rainfall for a long time prior to runoff. The data of precipitation and runoff amount from 1997 to 2008 required to assess the original and improved SCS models were collected from the 3 drainage areas of 1600 m2, 0.06 km2 and 1.36 km2 in the northern part of the Huaihe River basin, China. As antecedent daily precipitation period was 5 d and initial abstraction coefficient equaled to 0.2, the least-squares estimation method was used to calibrate the model parameters, i.e. the effective rainfall influence coefficient and the curve number, and the percent bias (PBIAS), Nash-Sutcliffe efficiency (NSE) and coefficient of determination (R2) were utilized to compare and assess the performance of the original and improved SCS models. The improved SCS model predicted daily runoff amount more accurately than the original model, and the improved SCS model increased the R2 and NSE by 27.0%-30.9% and 1.0%-78.3%, respectively, compared with the original during the validation period. Both models were calibrated with the close curve number, the effective rainfall influence coefficient was relatively stable, and the coefficient variation of 25% at plot scale resulted in the runoff prediction variation of less than 5%. The improved SCS model would perform better if it is applied in the areas with high evapotranspiration and seepage. [ABSTRACT FROM AUTHOR]