Understanding the spatial and temporal dynamics of soil water and crop water stress within a field is critical for effective Variable Rate Irrigation (VRI) management. Proper VRI can result in improved protection of the crop from early onset of crop water stress while minimizing runoff and drainage losses. The objectives of this study are (1) to examine zone delineation for informing irrigation recommendations from volumetric water content (VWC) and field capacity (FC) to grow similar or greater wheat yields with less water, (2) evaluate the ability to model soil and crop water dynamics within a season and within a field of irrigated winter wheat, and evaluate the sensitivity of crop water stress, evapotranspiration and soil water depletion outputs within a water balance model with Penman-Monteith evapotranspiration (ET) in response to adjusted soil properties, spring volumetric water content (VWC), and crop coefficient model input values. Five irrigation zones were delineated from two years of historical yield and evapotranspiration (ET) data. Soil sensors were placed at multiple depths within each zone to give real time data of the VWC values within each soil profile. Soil samples were taken within a 22 ha field of winter wheat (Triticum aestivum ‘UI Magic’) near Grace, Idaho, USA multiple times during a growing season to describe the spatial variation of VWC throughout the field, and to assist in modeling soil water dynamics and crop water stress through energy balance and water balance equations. Spatial variation of VWC was observed throughout the field, and on a smaller scale within each zone, suggesting the benefit of breaking portions of the field into zones for irrigation management purposes. Irrigation events were triggered when soil sensors detected low values of VWC, with each zone receiving unique rates intended to refill to zone specific FC. Cumulative irrigation rates varied among zones and the VRI approach saved water when compared to an estimated uniform Grower Standard Practice (GSP) irrigation approach. This method of zone management with soil sampling and sensors approximately represented the VWC within each zone and proved beneficial with effective reduction of irrigation rates in every zone compared to an estimated GSP. As such, there was a delay in the premature onset of crop water stress throughout some areas of the field. Variability in soil properties and spring soil moisture were key in giving accurate values to the model in order to make proper VRI management decisions. When assessing the model sensitivity, changing the inputs such as FC, wilting point (WP), total available water (TAW), spring VWC and crop coefficient (Kc) by -4 to +4 standard deviations away from their spatially average values, impacted the outputs of the model, with Kc having a large impact all three of the outputs. Further work is needed to improve the accuracy of representing VWC throughout a field, thus improving VRI management, and there is potential benefit in using a variable crop coefficient could to more accurate VRI management decisions from a soil water depletion model.

Irrigated agriculture is the largest user of freshwater in a world experiencing increased water scarcity and water demands. Variable rate irrigation (VRI) aims to use water efficiently in crop production, resulting in good yields and water conservation. With VRI, the grower is able to employ custom irrigation rates for different parts of a field. Adoption of VRI has been limited due to the complexity of matching irrigation to spatiotemporal crop water needs and the cost/benefit economics of VRI equipment. The goal of this study was to quantify spatiotemporal variability of VWC in a field that has uniform soil type and discuss the driving factors that contribute to that variability. Soil samples were acquired at 66 and 87 locations during the 2019 growing season at two study sites. Soil samples from 32 and 48 locations within each study site were selected to be analyzed for soil texture properties. The USGS Web Soil Survey was also referenced. Both, the USGS data and the data collected for this project showed very uniform soils across both fields. The objectives of this study were i) to show variability of VWC within fields that contain uniform soil texture using univariate Local Moran’s I (LMI) and ii) to compare static VRI zones based on spatial patterns of readily available field data that might serve as surrogates for VRI zones created from measured variation of soil volumetric water content (VWC). Management zones created using readily available field data had reasonable correlations with VWC. In both study sites, elevation was found to be the best variable for delineating VRI zones that imitate measured VWC.

The Maputaland Coastal Plain in Kwa Zulu-Natal is covered by aeolian sands . According to the national wetland delineation manual delineating wetlands in this area is complex due to problematic wetland soils. The aim of this study was to classify and describe the vegetation of the different zones of the wetland systems to determine the main drivers of the plant communities. The understanding of these complex ecosystems could assist in wetland delineation. Twenty wetlands from five wetland systems were surveyed. The vegetation zones of each wetland were sampled and basic environmental data were collected. The results from this study indicate that different plant species assemblages are characteristic for the wetland zones. However, zone delineation using vegetation composition varies between the different wetland systems, and should be evaluated according to the system in question. The major determinants of the vegetation communities in wetlands on the MCP are the substrate type and hydrological regime / Environmental Sciences / M.Sc. (Environmental Management)

The Maputaland Coastal Plain in Kwa Zulu-Natal is covered by aeolian sands . According to the national wetland delineation manual delineating wetlands in this area is complex due to problematic wetland soils. The aim of this study was to classify and describe the vegetation of the different zones of the wetland systems to determine the main drivers of the plant communities. The understanding of these complex ecosystems could assist in wetland delineation. Twenty wetlands from five wetland systems were surveyed. The vegetation zones of each wetland were sampled and basic environmental data were collected. The results from this study indicate that different plant species assemblages are characteristic for the wetland zones. However, zone delineation using vegetation composition varies between the different wetland systems, and should be evaluated according to the system in question. The major determinants of the vegetation communities in wetlands on the MCP are the substrate type and hydrological regime / Environmental Sciences / M.Sc. (Environmental Management)