Modelling Shallow Groundwater Evaporation Rates from a Large Tank Experiment

A large tank (1.4 m x 4.0 m x 1.3 m) filled with medium-coarse sand was employed to measure evaporation rates from shallow groundwater at controlled laboratory conditions, to determine drivers and mechanisms. To monitor the groundwater level drawdown 12 piezometers were installed in a semi regular grid and equipped with high precision water level, temperature, and electrical conductivity (EC) probes. In each piezometer, 6 micro sampling ports were installed every 10 cm to capture vertical salinity gradients. Moreover, the soil water content, temperature and EC were measured in the unsaturated zone using TDR probes placed at 5, 20 and 40 cm depth. The monitoring started in February 2020 and lasted for 4 months until the groundwater drawdown became residual. To model the groundwater heads, temperature, and salinity variations SEAWAT 4.0 was employed. The calibrated model was then used to obtain the unknown parameters, such as: maximum evaporation rates (1.5-4.4 mm/d), extinction depth (0.90 m), mineral dissolution (5.0e-9 g/d) and evaporation concentration (0.35 g/L). Despite the drawdown was uniformly distributed, the increase of groundwater salinity was rather uneven, while the temperature increase mimicked the atmospheric temperature increase. The initial groundwater salinity and the small changes in the evaporation rate controlled the evapoconcentration process in groundwater, while the effective porosity was the most sensitive parameter. This study demonstrates that shallow groundwater evaporation from sandy soils can produce homogeneous water table drawdown but appreciable differences in the distribution of groundwater salinity.

Using Bromide Data Tracer and HYDRUS-1D to Estimate Groundwater Recharge and Evapotranspiration under Film-mulched Drip Irrigation in an Arid Inland Basin, Northwest China

Accurate estimation of groundwater recharge (GR) and evapotranspiration (ET) are essential for sustainable groundwater resources management, especially in water-limited arid and semi-arid regions. In the Manas River Basin (MRB), water shortage is the main factor restricting sustainable development of irrigated agriculture, which relies heavily on groundwater. Film-mulched drip irrigation significantly changes the pattern and dominant processes of water flow in the unsaturated zone, which increases the difficulty of estimating GR and ET. To better estimate GR and ET under film-mulched drip irrigation in the MRB, bromide tracer tests and soil lithologic investigation were conducted at twelve representative sites in the MRB. A one-dimensional variably saturated flow model (HYDRUS-1D) was calibrated at each site using groundwater evaporation data inferred from the bromide tracer tests. The results showed that average annual groundwater evaporation in uncultivated lands calculated from bromide trace tests was 25.55 mm. Good simulation accuracy was achieved between the observed and simulated evaporation. Model calculations showed that the annual GR was within 5.5 to 37.0 mm under film-mulched drip irrigation. The annual ET was within 507.0 to 747.1 mm, with soil evaporation between 35.7 to 117.0 mm and transpiration between 460.9 to 642.3 mm, respectively. The portion of the soil evaporation for the total ET was within 7% to 16% and more than 70% of the precipitation and irrigation water was used by the cotton plants. Spatial variations in soil lithology, water-table depth, and initial soil water content led to the spatial differences of GR and ET in the MRB. Our study indicated that bromide tracer tests may be used to evaluated ET in the arid and semi-arid oases. The combination of bromide tracer tests and one dimensional variably saturated model can enhance reliability for estimation of GR and ET under film-mulched drip irrigation not only in the MRB, Northwest China but also the other similar arid inland basins around the world.

Capillary rise and saliferous groundwater evaporation: effects of various solutes and concentrations

Capillary rise is capable of demonstrating the mechanism involved in groundwater evaporation, where the evaporation from saliferous groundwater could be quantized in accordance with fresh groundwater. The two types of experiments included 12 treatments with four solutes (KCl, NaCl, CaCl2, and MgCl2) that were dissolved in groundwater at three concentrations (5, 30, and 100 g/L), and one control treatment without the salt solutions. The results demonstrated that the capillary action played a dominant role only within a very short period of time at the beginning of evaporation (i.e. within 2 min). The total dissolved solids (TDS) of the groundwater that was dissolved with KCl or NaCl affected the capillary water gravity more than soil pore structure. The TDS of the groundwater that was dissolved with CaCl2 or MgCl2 affected both the capillary water gravity and the soil pore structure. During the groundwater evaporation process, the evaporation conversion coefficient CTDS (>1.0) had the potential to calculate the saliferous-groundwater evaporation in accordance with the fresh-groundwater evaporation. The CTDS values were the largest for the groundwater that was dissolved with KCl/NaCl and CaCl2/MgCl2 at 5 and 30 g/L, where it reached average values of 1.3530–1.3735 and 1.3257–1.3589.