| Dry drainage is suggested to be an effective technique to control soil salinity in an environmental friendly and cost-effective way.In this method,a part of land in irrigated system is permanently or seasonally fallowed.Groundwater table in neighboring irrigated croplands rises along with the irrigation events,whereas that in the fallow areas falls under high evaporative demand,resulting in hydraulic gradient and groundwater exchange from the croplands to the fallow areas.As a consequence,the excessive salt will eventually accumulate in the fallow areas and the salt balance in the croplands can be maintained.This work aims to quantify phreatic evaporation rate at fallow areas,as it is the dominant factor to evaluate the effectiveness and sustainability of dry drainage system.The interpretation of bromide tracer movement in unsaturated soil profile allows one to estimate phreatic evaporation,such field experiments were carried out at 50 sites located at fallow areas in Hetao irrigating area of Inner Mongolia over the past two years from 2017 to 2018.Furthermore,a water balance conceptual model and a Richards-based numerical model SWAP were established and calibrated to assess the use of bromide tracer method for in situ studies of phreatic evaporation.On the basis of tracer experiment results,a relation between dry-drainage capacity with the depth of groundwater,soil salt content,soil texture and the distance to the neighboring cropped land can be established.The analysis of phreatic evaporation laws on fallow lands are intended to provide reliable theoretical reference for the efficient and sustainable application of dry drainage system.The main conclusions are summarized as follows:1)Bromide tracer experiment was applied to evaluate the groundwater evaporation at different sampling sites.The average phreatic evaporation rate and evaporation coefficient determined by bromide tracer is 0.71mm/d and 0.17 during the experimental period for 2017;while for 2018,the average phreatic evaporation rate and evaporation coefficient is0.58mm/d and 0.16,respectively.This results are in good agreement with water balance method and numerical method,indicating bromide tracer method is an effective tool for determining phreatic evaporation fluxes in arid-semiarid regions.2)Two tracer sampling methods(the free-face method and the five-point method)and multiple injection depths(40cm,60 cm,80cm,and 100cm)were evaluated to verify the practicability of tracer experiment.Compared with the free-face method for bromide tracer experiments,the five-point method is easier to obtain a more evident peak of tracer’s concentration in soil profile,thus enhancing accuracy and effectiveness of phreatic evaporation assessments.Besides,the tracer injection depth is suggested to be as deeper as possible,the closer the injection depth to groundwater table,the more accurate phreatic evaporation flux we might obtain.3)Based on the results of bromide tracer experiment,the relationships between dry-drainage capacity with the depth of groundwater,soil salt content,soil texture and the distance to the neighboring irrigation lands were analyzed.Results show that the dry drainage capacity of the fallow areas is negatively correlated with groundwater depth and distance from irrigation area,indicating that there is an effective range for the fallow areas to evaporate water and store salt.4)The observations of soil water and salt dynamics showed that the fallow areas act as a sink for excess water and carrying the salt from the irrigated soils,where the water evaporates,leaving the salt to accumulate in the fallow area.It is calculated that over 70%of soil salt content is stored in the surface of soil profile,the effect of high level of salinity in soil surface is shown to decrease the dry-drainage flux during its long run.To maintain the sustainability of dry drainage system,there is an allowable limitation of soil salinity under certain groundwater depth for the fallow areas.This work proved that dry drainage is a viable alternative in controlling soil salinity of irrigated croplands in Hetao irrigating district.However,some limiting factors that influence the design have to be considered.These are related to the phreatic evaporation rate,groundwater depth,soil salt accumulation and soil properties of the fallow areas. |
