MONITORING NEAR-SURFACE SOIL-WATER STORAGE IN TURFGRASS USING TIME-DOMAIN REFLECTOMETRY AND WEIGHING LYSIMETRY

Monitoring near-surface soil water content with time domain reflectome try (TDR) could improve our understanding of water movement in a daily irrigated crop. But, TDR field validation is needed using weighing ly simeters to assess actual changes in soil water storage. In this study , we test the hypothesis that vertically installed TDR probes can be u sed to measure evapotranspiration (ET) in a full-cover turfgrass envir onment. This was done by directly comparing changes in soil water stor age measured with TDR against changes in mass measured with weighing l ysimeters. The TDR probes (200, 400, 600, and 800 mm long) were instal led vertically in two weighing lysimeters. The TDR water content and l ysimeter mass determinations were made every 30 min for a 28-d period during daily irrigation intervals, and during a 6-d irrigation interva l. Average water contents increased with increasing probe length; e.g. , 600- and 100-mm probes yielded average water contents of 18.1 and 18 .5%, vs. 14.8% for the 200-mm probe. These differences reflect the lac k of transpirational uptake from deeper soil layers. All TDR probes un derestimated water added and lost compared with the lysimeter during d aily irrigation, with shorter probes exhibiting larger differences tha n longer probes. However, when the irrigation interval was increased t o 6 d, the longer TDR probes were more suited to measuring water loss; for example, the 200-, 400-, 600-, and 800-mm probes measured 40, 77, 78, and 96% of the water loss recorded by the lysimeter, which only r ecords ET. This shows that increasing amounts of water were taken up f rom deeper layers as the soil dried.