The Aksu Region, located in southern Xinjiang, is one of the major areas in China that produces cotton. The main climate feature in this arid region is the extremely high ratio of evaporation to precipitation. Water scarcity is one of the most critical constraints for sustainable cotton production. Drip irrigation under mulch is considered as the most efficient water-saving method because this system can uniformly distribute water, accurately control the amount of applied water, and reduce evaporation [1
]. Hence, this method is widely practiced in semi-arid and arid regions [3
]. In Xinjiang, drip irrigation under mulch is the main water-saving irrigation method and has been applied to cotton cropping systems since 1996. The area of drip-irrigated farmlands reached 15.3 million ha in 2009 [5
Considerable progress has been made to understand water flow, solute transport, and root water uptake under drip irrigation covered by mulch. The characteristics of cotton root distribution under different amounts of irrigation have been studied [5
]. Fang et al.
] reported that cotton root is mainly distributed in the 0–40 cm soil layer, and its biomass in the deep soil layer increases with the decrease in irrigation amount. Yan et al.
] determined a significant correlation between cotton root length density and biomass. According to their experiment results, maximum cotton production was 6360.8 kg·ha−1
, which corresponds to an irrigation amount of 3464.4 m3
]. The distribution of water and salt affected by drip irrigation under mulch is also a critical issue that has been studied in various experiments [1
However, the effects of drip irrigation under mulch on soil water movement and root zone water balance components have not been fully studied. Plastic mulch is an important factor that influences soil moisture, soil temperature, and surface microclimate [18
]. Moreover, several studies have indicated that plastic mulch has a greater effect on soil water balance and ultimately improves water use efficiency [19
]. Thus, the effects of mulch on soil moisture and root zone water balance under drip irrigation must be examined for the proper design, management, and operation of drip irrigation systems.
Time- and site-specific experiment studies, which have been conducted in this region, can provide information for irrigation management under specific soil–water–plant system and given climate conditions. However, this information may not be applicable to other regions or to the same region in other years because of the high variability of field and climate. Therefore, a model, as a common management tool, is required to guide irrigation practice in different regions. In addition, measuring all soil water balance components in a field experiment is difficult, if not impossible. A modeling approach coupled with field experiments can determine all components of water balance, including root water uptake, drainage, and recharge from low soil profiles.
The soil moisture pattern under drip irrigation has been studied using empirical, analytical, and numerical models in the past decades [22
]. HYDRUS-2D is a hydrologic model that simulates water, heat, and solute movements in 2D and 3D variably saturated porous media [29
]. HYDRUS-2D has been successfully used to model various solute transports under drip irrigation, such as nitrogen leaching, fertigation, and salt accumulation [31
]. Previous simulation studies [29
] showed that HYDRUS-2D simulations of soil water content are in reasonable agreement with measured values.
In the present work, a field experiment and a simulation study were conducted. The measured data in the field experiment were used to calibrate, validate, and apply the HYDRUS-2D model. Then, the model was used to quantify the effects of mulch and various amounts of drip irrigation on soil water distribution and root zone water balance components, which is essential to evaluate and manage drip irrigation systems in this region.
The objectives of this study were to (1) determine a modeling approach that can be applied in this region for drip irrigation management under mulch and (2) evaluate the effects of different amounts of drip irrigation under mulch on soil water distribution pattern and water balance components in a cotton cropping system.
For this research, field irrigation experiments with four treatments were conducted. The objective of these experiments was to evaluate the accuracy of the demonstrated model for simulating the soil water dynamic under drip irrigation with mulch. Besides this objective, an analysis of the role played by mulch on soil water balance components was conducted. For this purpose, additional numerical experiments were carried out.
The simulation model (HYDRUS-2D) was calibrated, validated, and applied in a cotton field irrigated by a drip irrigation system under mulch cover. The results suggest that the observed soil water content and the simulated results obtained with HYDRUS-2D are in good agreement. Root water uptake, evaporation, and the radius of the wetting zone decreased as the amount of applied irrigation was reduced (from T1 to T4). Soil water storage and recharge from low soil profiles were increased to compensate for the difference in irrigation. The stored and recharged soil water from low soil layers are highly important water sources, accounting for 3.6%, 15.6%, 27.6%, and 31.1% of water losses for T1, T2, T3, and T4, respectively.
The simulation results revealed that mulch has a minimal effect on soil water distribution patterns. However, mulch is effective for soil water conservation. Evaporation was significantly increased from 3.4 to 25.1 mm after mulch was removed, whereas root water uptake was decreased by 11.2 mm. Drip irrigation under mulch is a widely applied water-saving irrigation technology that can reduce evaporation, improve water use efficiency, and conserve water in extremely arid regions.
In terms of the analysis of the 2D distribution patterns of soil water and water balance components, the HYDRUS-2D model can be applied to assist in the design and development of management practices for drip irrigation systems under mulch cover in arid regions.