Submitted to: Irrigation Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/3/2005
Publication Date: 10/21/2005
Citation: Hunsaker, D.J., Pinter Jr, P.J., Kimball, B.A. 2005. Wheat basal crop coefficients determined by normalized difference vegetation index. Irrigation Science. 24:1-14. Interpretive Summary: Dependable estimates of the amount of water used in crop evapotranspiration (ET), which is the combined crop transpiration and evaporation from the soil, are needed for determining the proper timing and amounts for crop irrigations. Crop coefficient (Kc) curves have been developed for many different crops and are widely used in irrigation scheduling to estimate the daily ET use from planting through harvest. However, the Kc curves that are presently available are fairly generalized and often fail to give good estimates of the actual daily ET use, which can lead to improper irrigation scheduling and poor water management. For example, generalized Kc curves presently cannot be routinely used to account for crop ET changes caused by differences in crop development due to nutrient deficiencies or insect pressures. This study presents a method to estimate daily crop coefficient values for wheat from remotely sensed measurements of crop reflectances, which can indicate differences in crop development and can be routinely measured either on the ground, in the air, or by satellite. The Kc estimated from the crop reflectance is expected to provide an estimate of crop water use based on the actual crop condition. This technique potentially offers wheat farmers, irrigation consultants, and government agencies a means to realistically determine daily ET occurring within a wheat field.
Technical Abstract: Crop coefficient methodologies are widely used to estimate actual crop evapotranspiration (ETc) for determining irrigation scheduling. Generalized crop coefficient curves presented in the literature are limited to providing estimates of ETc for 'optimum' crop condition within a field, which often need to be modified for local conditions and cultural practices, as well as adjusted for the variations from normal crop and weather conditions that might occur during a given the growing season. Consequently, the uncertainties associated with generalized crop coefficients can result in ETc estimates that are significantly different from actual ETc, which could ultimately contribute to poor irrigation water management. Some important crop properties such as percent cover and leaf area index have been modeled with various vegetation indices (VIs), providing a means to quantify real-time crop variations from remotely-sensed VI observations. Limited research has also shown that VIs can be used to estimate the basal crop coefficient (Kcb) for several crops, including corn and cotton. The objective of this research was to develop a model for estimating Kcb values from observations of the normalized difference vegetation index (NDVI) for spring wheat. The Kcb data were derived from back-calculations of the FAO-56 dual crop coefficient procedures using field data obtained during two wheat experiments conducted during 1993-4 and 1995-6 at Maricopa, Arizona. The performance of the Kcb model for estimating ETc was evaluated using data from a third wheat experiment in 1996-7, also at Maricopa, Arizona. The Kcb was modeled as a function of a normalized quantity for NDVI, using a third-order polynomial regression relationship (r2 = 0.90, n = 232). The estimated cumulative ETc for the 1996-7 season agreed to within -33 mm (-5%) to18 mm (3%) of measured ETc. However, the mean absolute percent difference between the estimated and measured daily ETc varied from 9% to 10%, which was similar to the 10% variation for Kcb that was unexplained by model. The preliminary evaluation suggests that remotely-sensed NDVI observations could provide real-time Kcb estimates for determining the actual wheat ETc during the growing season.