Abstract

Throughout the world, there continues to be increasing demand for freshwater resources. With urban expansion, increased demand from a growing population, drought, and climate change, growers must produce more crops using less water. One approach to water conservation is with a variable rate irrigation (VRI) system. VRI technology allows growers to irrigate crops according to site-specific within-field variation instead of a uniform rate across the field. This thesis addresses aspects of within-field variation of irrigated alfalfa as it relates to the potential for VRI. Chapter 1 examines measurement of within-field variation of leaf area index (LAI) to inform variable-rate irrigation (VRI) for irrigated alfalfa (Medicago sativa). LAI is useful for predicting zone-specific evapotranspiration (ETc). One approach toward estimating LAI is to utilize the relationship between LAI and visible vegetation indices (VVIs) from unmanned aerial vehicle (UAV) imagery. This research has three objectives: (1) to measure and describe the within-field variation in LAI and canopy height for an alfalfa field under uniform irrigation, (2) to evaluate the relationships between the alfalfa LAI and various VVIs with and without field average canopy height, and (3) to use UAV images and field average canopy height to describe the within-field variation in LAI and the potential application to VRI. There was strong spatial clustering in the measured LAI but the spatial patterns were dynamic between dates. Among eleven VVIs evaluated, four that combined green and red wavelengths but excluded blue wavelengths showed the most promise. For all VVIs, adding average canopy height to multiple linear regression improved LAI prediction. The regression model using the modified green-red vegetation index (MGRVI) and canopy height (R2 = 0.93) was applied to describe the spatial variation in the LAI among VRI zones. There were significant (p < 0.05) but not practical differences (<15%) between pre-defined zones. UAV imagery coupled with field average canopy height can be a useful tool for predicting LAI in alfalfa. Chapter 2 details a study conducted to determine whether alfalfa is a good candidate crop for VRI by evaluating within-field variation of crop water productivity (CWP) under uniform irrigation. Evaluation of spatial patterns of CWP in combination with biomass yield has potential for delineating VRI zones and indicating whether to increase or decrease irrigation rates through VRI. It was hypothesized that spatial variation in CWP would justify the use of VRI due to alfalfa's high irrigation water demand, deep root system, and known spatially variable responses to soil types and depths. The objectives of this uniform irrigation alfalfa study were: (1) to measure within-field variation of crop evapotranspiration (ET), (2) to quantify within-field spatial variation of biomass yield, and (3) to describe the within-field spatial variability of CWP and how it can be clustered together with biomass yields into potential VRI management zones. The study was conducted on a 22.6 ha center-pivot irrigated alfalfa field in Idaho, USA for three harvest intervals in 2021 and 2022. ET was determined at 66 individual sample points using a water balance method and soil water sampling. Over the full study and all sample locations, ET was highly variable and ranged from 0.36 to 0.89 cm d-1. ET variation was significantly clustered (p < 0.05) for each harvest interval, but patterns were not consistent among harvest intervals. Biomass yield also varied significantly, ranging from 2.1 to 9.7 Mg ha-1, and had significant spatial clustering (p < 0.05). Using a bivariate Moran's I classification, spatial patterns of biomass yield-CWP show potential for VRI zone delineation. Using this bivariate delineation approach for the uniformly irrigated field, an average of 33% of the field was classified as over-watered and 18% of the field as under-watered. To illustrate the potential application of VRI, a 10% increase in irrigation rates in the under-watered areas and a 10% reduction in irrigation rates in the overwatered areas would result in a modest 1% reduction in irrigation application while improving CWP from more site-specific irrigation. This illustrates that a potential value for VRI is improved crop water productivity through redistribution of irrigation water within a field.

Degree

MS

College and Department

Life Sciences; Plant and Wildlife Sciences

Rights

https://lib.byu.edu/about/copyright/

Date Submitted

2023-08-15

Document Type

Thesis

Handle

http://hdl.lib.byu.edu/1877/etd13361

Keywords

variable-rate irrigation, leaf area index; unmanned aerial vehicle; alfalfa; crop water productivity; visible vegetation index; management zones

Language

english

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