Modelling soil water dynamic in rain-fed spring maize field with plastic mulching
Introduction
Rain-fed agriculture, makes up approximately 80% of global cropland and produces 60–70% of the world’s food (Falkenmark and Rockström, 2004; Rost et al., 2008). It plays a dominant role in the global food supply, especially considering the increasing global water shortage (Rockström et al., 2010). In China, rainfed agriculture accounts for approximately 25 Mha of the arable land and is mainly located on the semi-arid Loess Plateau and in northeast China, where the crop yields are limited by the soil water deficit and the low soil temperatures in the spring (Deng et al., 2006; Xiao et al., 2016). Plastic mulching has become a popular agricultural technology in those area, since it is thought to maintain soil moisture and increase the soil temperature (Tarara, 2000; Kader et al., 2017). For economic reasons, farmers usually do not cover the entire field with plastic film, but use polyethylene (PE) plastic strips which are alternated with bare strips, a technique called partial plastic mulching.
Soil water dynamics in fields with plastic mulching has received wide attention (Fisher, 1995; Wu et al., 2017; Ren et al., 2017; Dong et al., 2009; Zhao et al., 2012), since not only crop performance, but also environmental processes such as nitrate leaching and the emission of greenhouse gasses depend heavily on soil moisture dynamics (Qin et al., 2015, Filipovic et al., 2016, Liu et al., 2016). Measurement methods such as gravimetric soil water determination on soil samples and time domain reflectometry, and simulations have been used to obtain information on soil water dynamics and distribution (Wu et al., 2017; Ren et al., 2017; Filipovic et al., 2016; Li et al., 2015, Liu et al., 2013). Compared to field measurements, modelling studies are low-cost and present a high temporal and spatial resolution.
Models that can be used to obtain soil water information mainly include WaSim-ETH, Community Land Model, SiSPAT-Isotope, Hydrus and crop models such as SWAP, CERES, WOFOST and Aquacrop (Vereecken et al., 2016). Hydrus is mainly used at the pedo up to field scale. Crop models are mainly applied at field scale or regional scale, while the WaSim-ETH, Community Land Model, SiSPAT-Isotope are mainly applied at catchment scale or landscape scale. In fileds with plastic mulching, the different water infiltration and evaporation characteristics of plastic strips and bare strips may lead to obvious inhomogeneous soil water distribution. However, the soil water transport modules in most crop models are one-dimensional, and they are un-able to describe the spatial variation of soil water. Soil hydraulic model such as Hydrus-2D model can solve this problem by applying different type of boundary conditions for the plastic strip and bare strip. For these reasons, the Hydrus-2D model was adopted for the current study.
Hydrus-2D has been widely used to simulate soil water dynamic in irrigated field with plastic mulching, but not yet under rain-fed conditions. Different from the irrigated field, rainfall is the only source of soil water in the rain-fed field and a good understanding of rainfall infiltration processes is decisive for a correct prediction of the spatio-temporal patterns of soil water. In case of partial plastic mulching, rainfall can infiltrate into the soil through three pathways: (i) interception by maize leaves and transfer along the stem into the planting hole (i.e. canopy redistribution). Rainfall that is not intercepted by the maize leaves will (ii) reach the ground directly and infiltrate into the bare soil, or (iii) reach the surface of the plastic film and flow towards the bare soil and infiltrate into the soil at the film side (film-side infiltration)(Chen et al., 2017). Previous studies have applied two approaches to represent the rainfall infiltration in irrigated fields with plastic mulching: (i) In arid regions (annual rainfall amount < 200 mm), Han et al. (2015), Li et al. (2015) and Liu et al. (2013) simplified reality by neglecting canopy redistribution and film side infiltration. They assumed that rainfall directly reached the bare strip and infiltrated there (without canopy redistribution) and they omitted rainfall that reached the plastic strip; (ii) In the humid regions (annual rainfall amount > 800 mm), Filipovic et al. (2016) and Dusek et al. (2010) also neglected canopy redistribution, but integrated the process of film side infiltration by increasing the rainfall infiltration amount in bare strip with a factor which was equal to the ratio of plastic strip width to bare strip width. However, the performance of those two methods has not been evaluated under rain-fed conditions, where the redistribution may play an even bigger role, since no additional water is added under the plastic sheets through irrigation tubes.
In this study, we developed and applied a modelling strategy taking into account of both canopy redistribution and film side infiltration in field with plastic mulching. The objectives of this study were therefore (i) to compare the performance of different simulation strategies on reproducing soil water dynamic in rain-fed field with plastic mulching; (ii) to quantify the spatial variation of the SWC in rain-fed field with plastic mulching using the optimized simulation strategy. We hypothesized that rainfall redistribution and film side infiltration play important roles in soil water dynamics, and are not negligible under rain-fed conditions. In this study, the performance of Hydrus-2D was tested with 2 years of field data from a spring maize (Zea may L.) field that located on the Loess Plateau of China. We chose to focus on maize, since it was one of the main crops in the studied region and of great importance for Chinese agriculture (38.1 million hm2 in China in 2015 (NBSC (National Bureau of Statistics of China), 2016)).
Section snippets
Research site
The research site (37°45′N, 113°12′E, 1202 m altitude) is located at the Shouyang County which belongs to Shanxi Province, on the eastern part of the Loess Plateau, about 500 km west from the China’s capital Beijing. The majority of the farmland in Shouyang County produces crops, especially maize, soy bean and potato. The research area is characterized by a semiarid temperate continental monsoon climate with four distinct seasons. According to the weather record from Shouyang weather station,
Comparison of the performance of different treatment methods for rainfall infiltration in PM
Fig. 3 shows the simulated and measured SWC in different soil layers and positions, and Table 3 shows the corresponding RMSE and RRMSE. The BP scenario did not reproduce the water content peaks caused by concentrated rainfall at the middle of plastic strip in both years. The performances improved with depth, but in general the RRMSEs were poor ( >0.3) and therefore this scenario was unsuitable to represent the field context. At the middle of bare strip, simulation BP was doing better than under
Discussion
Our results suggest that the performance of Hydrus-2D in rain-fed field with plastic mulching depends strongly on the way the incoming rainfall is partitioned over the different types of surface boundaries and whether or not canopy redistribution is taken into account. Previous researches in arid regions suggested that Hydrus-2D could simulate the soil moisture dynamics well without taking into account these processes (Han et al., 2015; Li et al., 2015; Liu et al., 2013), probably because in
Conclusion
In the present study, we tested the performance of three strategies to represent rainfall infiltration in rain-fed fields with partial plastic mulching and applied the optimized strategy to investigate the spatial variation of soil moisture. The comparison demonstrated that when rainfall canopy redistribution and film side infiltration were neglected, Hydrus-2D failed to reproduce the soil water dynamics in all soil layers in the plastic strip and in the deep soil layers (20–60 cm and
Acknowledgements
This work was supported by the National Natural Science Foundation of China [grant numbers: 31370522; 31661143011], and the Special Fund for Agro-scientific Research in the Public Interest [grant numbers: 201503120]. We thank the University of Liège-Gembloux Agro-Bio Tech and more specifically the research platform Terra Research & Teaching Center for the scientific stay in Belgium that made this paper possible. We thank the scholarship “Erasmus+ International Credit Mobility” awarded by the
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2019, Soil and Tillage ResearchCitation Excerpt :Although numerical simulations of mulching and different soil conservation practices with HYDRUS-1D have been reported by several investigators (Dahiya et al., 2007; Zhao et al., 2010; Kodešová et al., 2014; González et al., 2015; Liang et al., 2017), none of them modeled straw mulching. Previous studies that quantified the effects of plastic mulching with HYDRUS software can be categorized into: changing of boundary conditions (Chen et al., 2018; Saglam et al., 2017), reducing of crop coefficient (Li et al., 2015), separating of potential evaporation and transpiration (Chen et al., 2014; Zhang et al., 2018), reducing of rainfall by a fraction (Zhao et al., 2018) and modifying of energy balance (Liang et al., 2017). These simulation studies were limited to plastic mulch only, and most of them were unable to predict the vapor and heat flows.