HYDRUS-1D Simulation of Soil Water Dynamics and Response of Different ETo Models to Crop Evapotranspiration (ETc) Under a Rainfed Condition in Southern Manitoba

Soil water content (SWC) plays a critical role in crop yield, irrigation scheduling, and water resources management. In the Canadian Prairies, the water content in the rootzone replenished by rainfall is rarely sufficient to satisfy crop water requirements. Thus, the need for robust and effective water management. Hydrologic modelling provides the opportunity to understand the underlying processes controlling and affecting soil water movement and distribution. Evapotranspiration (ET) is an important input of hydrologic models; thus, the estimation of ET could have significant consequences on modelling outcome and inference. The FAO Penman-Monteith (PM) is the recommended model for estimating the reference crop evapotranspiration (ETo). However, it is limited by requiring too many weather variables that are not readily available. Simple empirical ETo models have been developed as an alternative. In this study, six ETo models with different inputs were used to simulate soil water dynamics in a rainfed potato farm in Winkler, Manitoba, using the HYDRUS-1D model. The results showed that when used to simulate SWC, all the models followed a similar pattern, although a significant difference was observed at shallow depth (20 cm). Specifically, a significant difference (p < 0.05) was observed between observed and simulated SWC from Hargreaves Samani, Romanenko, Penman, and FAO-PM (missing) models. When used to simulate the crop evapotranspiration (ETc), there was no significant difference (p > 0.05) between observed and simulated ETc from FAO PM, Irmak, and Priestly – Taylor models. Hence, ETo models with fewer data inputs such as Irmak and Priestly – Taylor models can provide accurate and reliable results for water management in southern Manitoba.

Nitrogen and Rainfall Effects on Crop Growth—Experimental Results and Scenario Analyses

Nitrogen (N) fertilization is critical for crop growth; however, its effect on crop growth and evapotranspiration (ETc) behaviors under different amounts of rainfall is not well understood. As such, there is a need for studying the impact of nitrogen application rates and rainfall amounts on crop growth and ETc components. Agricultural system models help to fill this knowledge gap, e.g., the Root Zone Water Quality Model (RZWQM2), which integrates crop growth-related processes. The objective of this study is to investigate the effect of the nitrogen application rate on crop growth, soil water dynamics, and ETc behavior under different rainfall amounts by using experimental data and the RZWQM2. A field study was conducted from 2016 to 2019 with three nitrogen application rates (0, 70 and 130 kg N ha−1) for unirrigated winter wheat (Triticum aestivum L.), and two nitrogen application rates (0 and 205 kg N ha−1) for unirrigated corn (Zea mays L.). For the period of 1986–2019, the amounts of actual rainfall during each crop growth period are categorized into four groups. Each rainfall group is used as a rainfall scenario in the RZWQM2 to explore the interactions between the rainfall amounts and N levels on the resulting crop growth and water status. The results show that the model satisfactorily captures the interaction effects of nitrogen application rates and rainfall amounts on the daily ETc and soil water dynamics. The nitrogen application rate showed a noticeable impact on the behavior of soil water dynamics and ETc components. The 75% rainfall scenario yielded the highest nitrogen uptake for both crops. This scenario revealed the highest water consumption for wheat, while corn showed the highest water uptake for the 100% rainfall scenario. The interaction between a high nitrogen level and 50% rainfall yielded the highest water use efficiency, while low nitrogen and 125% rainfall yielded the highest nitrogen use efficiency. A zero nitrogen rate yielded the highest ETc and lowest soil water content among all treatments. Moreover, the impacts of the nitrogen application rate on ETc behavior, crop growth, and soil water dynamics differed depending on the received rainfall amount.

Numerical Analysis of Soil Water Dynamics during Spinach Cultivation in a Soil Column with an Artificial Capillary Barrier under Different Irrigation Managements

Artificial capillary barriers (CBs) are used to improve root zone conditions as they can keep water and nutrients in the root zone by limiting downward percolation. Numerical analysis is one of the promising tools for evaluating CB systems’ performance during the cultivation of leafy vegetables. This study aims to investigate the effects of the CB system on soil water dynamics during spinach cultivation in a soil column under different irrigation scenarios using HYDRUS (2D/3D) by comparing uniform (UNI), line-source (LSI), and plant-targeted (PTI) irrigations combined with alternative irrigation schedules. Simulation results of volumetric soil water contents were generally corresponding to measured data. Simulation results with various hypothetical irrigation scenarios exhibited that the CB was an effective system to diminish percolation losses and improve the root zone’s soil water storage capacity. On the other hand, evaporation loss can be increased as more water is maintained near the surface. While this loss can be significantly minimized by reducing the water application area, the irrigation amount must be carefully defined because applying water in a smaller area may accelerate downward water movement so that the water content at the CB interface can reach close to saturation. In addition to the malfunction of the CB layer, it may also cause a reduction of plant root water uptake (RWU) because the root zone is too wet. Among evaluated irrigation scenarios, irrigating every two days with PTI was the best scenario for the spinach as water use efficiency was greatly improved.

Effects of the “Grain for Green” Program on Soil Water Dynamics in the Semi-Arid Grassland of Inner Mongolia, China

The Grain for Green Program (GGP) initiated by Chinese government significantly impacts mitigating environmental degradation. Soil water resources probably constrain large-scale vegetation restoration projects in arid and semi-arid regions. Characterizing soil water dynamics after the GGP’s implementation is essential in assessing whether vegetation restoration can be sustained as part of ecological restoration. In this study, four sites were selected for field investigation: original natural grassland (NG) and grassland that was reconverted from cropland 12 years (12-year site), 8 years (8-year site), and 6 years (6-year site) before. Soil water at five depths was measured continuously at 10 min intervals at four sites. The findings showed that less rainfall infiltrated a deeper soil layer as the time after restoration augmented, and the 12-year site had the shallowest infiltration depth and soil water storage. Younger restored grassland (8-year and 6-year sites) had a higher soil water content than older restored grassland (12-year site) and NG. The soil water content decreased steadily with restoration age after an immediate initial rise, and the highest soil moisture was in the 8-year site. The results suggest that soil water dynamics varied with GGP and a soil water deficit could be formed after the GGP’s implementation for 12 years in semi-arid grassland.

<i>STH-net:</i> a soil monitoring network for process-based hydrological modelling from the pedon to the hillslope scale

The Schäfertal Hillslope site is part of the TERENO Harz/Central German Lowland Observatory, and its soil water dynamics are being monitored intensively as part of an integrated, long-term, multi-scale, and multi-temporal research framework linking hydrological, pedological, atmospheric, and biodiversity-related research to investigate the influences of climate and land use change on the terrestrial system. Here, a new soil monitoring network, indicated as STH-net, has been recently implemented to provide high-resolution data about the most relevant hydrological variables and local soil properties. The monitoring network is spatially optimized, based on previous knowledge from soil mapping and soil moisture monitoring, in order to capture the spatial variability in soil properties and soil water dynamics along a catena across the site as well as in depth. The STH-net comprises eight stations instrumented with time-domain reflectometry (TDR) probes, soil temperature probes, and monitoring wells. Furthermore, a weather station provides data about the meteorological variables. A detailed soil characterization exists for locations where the TDR probes are installed. All data have been measured at a 10 min interval since 1 January 2019. The STH-net is intended to provide scientists with data needed for developing and testing modelling approaches in the context of vadose-zone hydrology at spatial scales ranging from the pedon to the hillslope. The data are available from the EUDAT portal (https://doi.org/10.23728/b2share.82818db7be054f5eb921d386a0bcaa74, Martini et al., 2020).