Precise irrigation scheduling for turfgrass using a subsurface electromagnetic soil moisture sensor

Instrumented weather stations are often used for evapotranspiration (ET) determination in order to estimate crop water use for irrigation scheduling. A direct estimate of crop water use by subsurface measurements of soil water content has been limited by the high cost of reliable soil moisture sensors. Recent advances in electromagnetic (EM) sensor technology have made automated irrigation scheduling a reality using state-of-the-art soil moisture sensing. our objectives were: (i) to compare irrigation scheduling in turfgrass based on weather station ET estimates with those from a novel time domain transmission (TDT) soil moisture sensor and (ii) to apply a computer-based numerical model to simulate volumetric soil water content (theta) dynamics at the burial depth of the sensor and any drainage occurring below the turfgrass rooting depth. The TDT sensor was designed to directly connect to a custom irrigation controller or to interface a small display/control box with a conventional irrigation timer where irrigation scheduling is based on a threshold water content (theta(Thresh)). The sensor circuitry controlled the irrigation schedule by allowing the preprogrammed schedule to operate whenever the sensor-estimated water content (theta(Sensor)) dropped below theta(Thresh). The TDT sensor was installed under Kentucky bluegrass with a nearby weather station providing estimates of ET for comparison over a period of approximately 7 weeks. The HYDRUS-2D numerical simulation model was used for predicting water content (theta(Sim)) in the soil profile. The model input requirements include the flow domain geometry and boundary conditions, along with estimates of evaporation, transpiration, precipitation, irrigation and root water uptake data. Relative to ET based irrigation recommendations, the TDT system applied approximately 16% less water when irrigating with a sprinkler having a relative application depth of 0.80 at the position of the sensor (i.e. fraction of the design application rate applied at the sensor). Relative to a fixed irrigation rate of 50 mm week(-1), the TDT system applied approximately 53% less water. Modeling results of the TDT sensor control indicated that no detectable water drained below the estimated 30 cm rooting depth of turfgrass when uncontrolled application events (e.g. rainfall) were ignored. Performance of the TDT system is dependent on the sensor burial depth and theta(Thresh). The theta(Thresh) value is soil-type dependent and should be established via consideration of theta at field capacity and permanent wilting point. The potential water savings with the TDT system is not only important to water conservation, but can save irrigators an estimated US$ 5.00-100.00 per month based on average water prices in the US and a 1000 m(2) irrigated turfgrass plot. (c) 2006 Elsevier B.V. All rights reserved.