Effect of Polyacrylamide-Based Water Retention and Brackish Water Mixed Application Rates on Soil Evaporation Characteristics

Under the condition of agricultural water resource scarcity, how to reasonably utilize the abundant brackish water resources in China is a problem that needs to be addressed. To reveal the effect of water retention agent mixing dosage on the evaporation characteristics of soil brackish water, three water retention agent mixing levels (mass percentages), namely, 0% (S0), 0.08% (S0.08), and 0.1% (S0.1), and four mineralization levels of irrigation water, namely, 0 g/L (W0), 1 g/L (W1), 3 g/L (W3), and 5 g/L (W5), were set up. The measured cumulative evaporation was fitted and analyzed using a soil evaporation model. Results showed that polyacrylamide (PAM)-based water retention agents coupled with brackish water had a promoting effect on soil water holding capacity; however, PAM-based water retention agents and brackish irrigation water were positively correlated with the proportion of soil with smaller and larger pores, respectively. Applying a PAM-based water retention agent narrowed the range of temperature fluctuations, especially under S0.08 in the 0–20-cm soil layers and under S0.1 in the 30–50-cm soil layers. Brackish irrigation water increased the daily temperature variation of the 0–10-cm surface soil, and brackish irrigation water was negatively correlated with the daily temperature variation of the deeper soil (20–50 cm). The peaks of daily temperature variation in the 0–20- and 30–50-cm soil layers were observed under W5 and W3, respectively. S0.08 had a certain inhibitory effect on the evaporation of the brackish water, whereas brackish water was irrigated alone, and the salinity of the water body was lower than 3 g/L, which would promote the cumulative evaporation of the soil. The Rose evaporation model is widely used when there is no continuous water supply at the lower boundary during evaporation, making it the optimal fitting model for soil evaporation under the conditions of this experiment. S0.08 inhibited salt accumulation on the soil surface and effectively promoted uniform salt distribution. Water retention agent mixing levels were positively correlated with soil electrical conductivity. Comprehensively considering the experimental results, the S0.08W3 was a more suitable strategy for utilizing brackish water resources in the arid and semiarid regions of northwestern China, which could provide a theoretical basis for optimizing agriculture water-saving measures and achieving soil sustainability in terms of physical quality.