ARS | Publication request: IRRIGATION SCHEDULING USING MOBILE TDR WITH A CHECKBOOK METHOD

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 18, 2000
Publication Date: N/A

Interpretive Summary: Improper irrigation scheduling can lead to over irrigation which in turn may cause agricultural chemical leaching to ground water. Currently, a checkbook type of accounting (plant available water equals incoming rain and irrigation, credits, minus crop water use, debits) is used by producers to schedule irrigations. This requires daily estimates of crop water use (evapo-transpiration, ET) which vary from soil to soil and field to field. With the recent availability of low-cost, mobile time domain reflectometry (TDR) instrumentation, plant available water in the soil can be measured instantaneously and accurately. Our research showed that inherent errors in estimations of ET used in the checkbook approach for irrigation scheduling can be corrected (updated) by periodic measurement of plant available water with mobile TDR. These corrections allow farmers and consultants to more precisely irrigate each specific field when needed, reducing the potential for over-irrigation (inefficient water use) with the associated probabilit of leaching agrichemicals into ground water by the application of excess water.

Technical Abstract: Time domain reflectomerty (TDR) has changed the way researchers measure soil water content. This technology can now be applied to practical field situations such as irrigation scheduling. Mobile TDR probes were inserted into the soil vertically and measured soil moisture in the upper 0.6 m of soil in two 0.3 m increments. Soil moisture measured by this method was comparable to that measured by a TDR with waveguides permanently installed in a horizontal position (1:1 relationship, r=0.92). The mobile TDR was used to measure soil moisture in many locations in three crops (corn, soybean, wheat) on selected days for two growing seasons. This information was used to update a checkbook method of irrigation scheduling and proved to be accurate and easy. Quick, mobile TDR measurements in several locations provided sound estimates of soil moisture throughout a field and allowed for enhanced water use efficiency and reduced leaching potential.