Diurnal and Seasonal Mapping of Water Deficit Index and Evapotranspiration by an Unmanned Aerial System: A Case Study for Winter Wheat in Denmark

Abstract

Precision irrigation is a promising method to mitigate the impacts of drought stress on crop production with the optimal use of water resources. However, the reliable assessment of plant water status has not been adequately demonstrated, and unmanned aerial systems (UAS) offer great potential for spatiotemporal improvements. This study utilized UAS equipped with multispectral and thermal sensors to detect and quantify drought stress in winter wheat (Triticum aestivum L.) using the Water Deficit Index (WDI). Biennial field experiments were conducted on coarse sand soil in Denmark and analyses were performed at both diurnal and seasonal timescales. The WDI was significantly correlated with leaf stomatal conductance (R2 = 0.61–0.73), and the correlation was weaker with leaf water potential (R2 = 0.39–0.56) and topsoil water status (the highest R2 of 0.68). A semi-physical model depicting the relationship between WDI and fraction of transpirable soil water (FTSW) in the root zone was derived with R2 = 0.74. Moreover, WDI estimates were improved using an energy balance model with an iterative scheme to estimate the net radiation and land surface temperature, as well as the dual crop coefficient. The diurnal variation in WDI revealed a pattern of the ratio of actual to potential evapotranspiration, being higher in the morning, decreasing at noon hours and ‘recovering’ in the afternoon. Future work should investigate the temporal upscaling of evapotranspiration, which may be used to develop methods for site-specific irrigation recommendations.

Determining Threshold Values for a Crop Water Stress Index-Based Center Pivot Irrigation with Optimum Grain Yield

“…For maize crops, the lowest T c (18.2 • C) appeared at the R6 stage, while the highest T c (37.4 • C) appeared at the V12 stage. Previous studies pointed out that too low T c might predict no stress [54,55]. The findings were also proven by our study in which CWSI was only 0.0-0.25 with significant low T c in over-wintering, which might result in an incorrect irrigation decision [56].…”

Spatiotemporal Winter Wheat Water Status Assessment Improvement Using a Water Deficit Index Derived from an Unmanned Aerial System in the North China Plain

“…In the Vegetation Index/Temperature (VIT) trapezoid ( Figure 2 ) it is essential to accurately estimate the fraction of vegetation cover in order to achieve high precision in the WDI calculations. Most articles used near-infrared based indices, such as NDVI (Normalized Difference Vegetation Index) and SAVI (Soil-Adjusted Vegetation Index) [ 5 , 14 , 15 , 16 , 20 ]. In this study, we investigated the use of several near-infrared-, red-edge-, and RGB-based indices in order to assess suitability for use in the WDI calculation.…”

“…Four theoretical cornets of the VIT trapezoid can be calculated by the following formula: where r a is aerodynamic resistance [sm −1 ], R n is net radiation [Wm −2 ], G is soil ground heat flux [Wm −2 ], C v is the volumetric heat capacity of air (1200 J °C −1 ), ρ is air density [kg m −3 ], VPD is vapor pressure deficit [kPa], γ is the psychrometric constant [0.067 kPa °C −1 ], ∆ is the slope of saturation vapor pressure curve [kPa °C −1 ], and r c is canopy resistance to vapor transport [s m −1 ]. Due to the differences in the conditions of each corner of the VIT trapezoid, r a , R n , G, and r c were calculated separately according to procedures described by Antoniuk et al (2021) [ 5 ]. The method accounts for the differences based on the vegetation cover as well as the differences between dry and wet bare soil.…”

“…More details about the calculation of all the specific parameters can be found in Moran et al (1994) [ 14 ] and Antoniuk et al (2021) [ 5 ]. The reference evapotranspiration rate was calculated using the Penman–Monteith equation [ 27 ]: where T a(hr) is mean hourly air temperature [°C]; u 2 is the wind speed at 2 m height [m s −1 ]; e 0 Ta(hr) is the saturation vapor pressure at the temperature T a [kPa]; e a is actual vapor pressure [kPa].…”

“…At a global scale, irrigation increases the attainable yield of winter wheat by almost 35%, while in the North China Plain (NCP) yield benefits due to irrigation are even higher due to greater evaporative demands [ 3 ]. The latest developed technologies, such as unmanned aerial platforms equipped with multispectral and thermal sensors (UAS), allow efficient field monitoring for the early detection of drought with faster response and special attention to be paid to spatial heterogeneity and patterns within the field [ 4 , 5 ]. However, a deeper understanding of plant physiological responses to drought and their interpretation by the sensors is needed in order to further improve UASs for advanced precision agriculture use.…”