Understanding Consumed Water at the Field Scale
Updated: Oct 18, 2022
Consumptive use of water by crops, often referred to as evapotranspiration (ET), is one of the most significant components of an agricultural region’s water balance. Thus, it is critically important for Groundwater Sustainability Agencies (GSAs) to understand consumed water for the implementation of Groundwater Sustainability Plans (GSPs) and compliance with the Sustainable Groundwater Management Act (SGMA).
The calculation of ET can be performed using a number of methods, all of which have differing levels of complexity, cost, and accuracy. Options range from empirical methods that utilize a crop coefficient and reference ET from weather stations, to the use of weighing lysimeters and eddy correlation monitoring techniques. These methods are limited, however, because they provide only point values of ET for a specific location and fail to provide the ET on a regional scale. This limitation has motivated the development of methods using remotely sensed (RS) data from satellites to evaluate ET over large areas. Satellite data are well suited for deriving spatially continuous ET surfaces that can be paired down to the field scale because of their temporal and spatial characteristics. However, the most accurate results from RS models requires calibration to actual and repeated surface measurements.
Having worked with a number of ET approaches and models over the years, Land IQ realized the need for accurate land use and calibration of ET estimates to actual cropped fields. As such, Land IQ developed a refined method for determining consumptive use of crops at a field or regional scale. The Land IQ ET data driven approach utilizes a variety of image and ground data sources and yields more accurate results than RS models alone because ground calibration data are available and included during the modeling process. The results are more accurate estimates of field-by-field ET as compared to other remote sensing methods.
The Land IQ ET model was developed for detailed, field-scale water use tracking. It uses robust ground station data and direct image analysis to interpret image data. The Land IQ ET model is differentiated from other models by the following:
Data-driven - The data driven approach makes the model completely objective. There is no need to hand-pick calibration pixels, as required in other models, which can introduce uncertainty, depending on analyst’s knowledge and experience.
Scale – Modeling is performed at the field level; thus, results can be aggregated to any larger unit of analysis desired.
Accurate and timely crop mapping – Land IQ produces and integrates highly accurate (97.6%) crop mapping of California, which it incorporates near real-time into its ET modelling efforts. These data are provided twice a year to ET clients.
Ground truth data – The Land IQ model integrates data from repeated and rigorous ground truth climatic and environmental stations (currently approximately 95). Field stations are distributed to correspond with the range and dominant crop types in the service area and are continuously monitored via telemetered systems to detect inconsistencies in collection or outages. Field station data is key for model calibration and to help quantify model accuracy.
Remote sensing imagery - Land IQ ET uses available remote sensing images (Landsat, Sentinel, and other purchased imagery, if needed). Typically, the model uses up to three times the imagery resources as compared to relying on Landsat alone.
Integration of agronomic features of modern cropping systems - Unlike other methods, the Land IQ data is derived from and guided by our understanding of agricultural systems, landscape processes, production systems, and crop phenology. In addition to basic land use data, the Land IQ ET model also considers and incorporates permanent crop age, permanent crop density, and unique field conditions including irrigation method and management where possible.
Land IQ ET was initially developed for use in the Semitropic Water Storage District (SWSD) beginning in 2016. Since that beginning and in addition to SWSD, the organizations utilizing Land IQ ET has grown to cover over 3 million acres including:
Over 40 GSAs or Irrigation Districts
With 35-40 different crop types
Utilizing multiple water sources
Click here to learn more about Land IQ ET.