Enhanced Soil Moisture Assessment using Narrowband Reflectance Vegetation Indices in Creeping Bentgrass

Abstract

Turfgrasses are measured aesthetically and by their ability to withstand stressors. Historically, researchers quantified acceptability by visual quality, but inconsistencies necessitate the use of vegetation indices (VIs) as an objective measurement. Indiscernible relationships have been established between turfgrass canopy normalized difference vegetation index (NDVI) and important variables such as soil moisture and leaf chlorophyll content. Alternative and variable–specific indices have been established in cropping systems. The water band index (WBI) is used for predicting water availability in cropping systems but has not been explored within turfgrass systems. The objective of this study was to compare the relationships of established VIs with chlorophyll content of ‘L‐93’ creeping bentgrass (Agrostis stolonifera L.) and water content of a sand‐based root zone maintained under greenhouse conditions. All VIs were moderately to strongly correlated to total chlorophyll content (r = 0.49–0.85). Turf color was more closely related to chlorophyll indices than to wet‐lab quantification. Only WBI (r ≥ 0.80) and the green/red ratio index (GRI; r ≥ 0.50) were consistently related to soil water content. The NDVI was weakly related to soil water content in one trial (r = 0.49). Nonlinear regression showed that WBI can be useful for estimating a decline in soil water content as water first becomes limiting for creeping bentgrass and may offer a viable means to assess water availability independently of non‐moisture‐related stresses and more accurately compared with previous indices. Future research will evaluate WBI on broader geospatial scales to assess practical application for turfgrass irrigation management.

Using Hyperspectral and Multispectral Indices to Detect Water Stress for an Urban Turfgrass System

“…Spectral reflectance sampling was conducted prior to mowing and taken to coincide with other turfgrass and soil measurements. Previous greenhouse turfgrass trials conducted by McCall et al [26] and Roberson [27] were used as the basis for determining indices to assess soil moisture status and turfgrass parameters ( Table 1). Both studies aimed to develop relationships of soil moisture status and health of turfgrass using various vegetation indices.…”

“…Spectral reflectance measurements have potential as a management tool for water and nutrient stress in turfgrass. McCall et al [26] and Roberson [27] demonstrated at the greenhouse scale the detection of water stress using spectral reflectance. However, variations in fertility and irrigation strategies, as well as the comparison of multispectral and hyperspectral radiometers, are largely unexplored at the field-scale in turfgrass research.…”

“…Despite the ability of the NDVI and RVI to quantify plant response to fertility or to stress (e.g., soil moisture), it is unable to differentiate and determine the cause of the plant response [16]. Water indices utilizing narrowband spectral reflectance have been used to assess plant water status and drought effects [26,37,38]. Water indices have potential for early water stress detection independent of other stressors.…”

“…Water absorption bands exist in the NIR region beyond the photosynthetically active radiation (PAR), reducing the confounding effect from other abiotic stresses. McCall et al [26] determined that soil VWC had the strongest correlation to the water band index (WBI) (r ≥ 0.80), Agronomy 2019, 9, 439 3 of 15 followed by the green-to-red ratio index (GRI) (r ≥ 0.50) in a greenhouse study assessing creeping bentgrass grown in a sand-based media:…”

“…Our research was to test the scalability of previous research conducted by McCall et al [26] and Roberson [27] relating spectral reflectance data to soil VWC and N fertility in a tall fescue (Festuca arundinacea Schreb.) urban turfgrass system.…”