Sensing Water from Subsurface Drip Irrigation Laterals: In Situ Sensors, Weighing Lysimeters and Cosmos Under Vegetated and Bare Conditions.

Characterization of soil water dynamics in the root zone under subsurface drip irrigated (SDI) is complicated by the three dimensional nature of water fluxes from drip emitters plus the fluxes, if any, of water from precipitation. In addition, soil water sensing systems may differ in their operating principles, and the applicable physics determines the volumetric extent of each measurement. Also, sensing volume decreases as water content increases for some sensor types. The time-varying overlap of the volumes of wetting and measurement can strongly affect the water contents reported from different sensing systems; and choice of sensor position vis-à-vis the drip tape affects the extent of this overlap. We used four different soil water sensing systems in two adjacent 4.66-ha fields irrigated by SDI: Two large (3 m by 3 m by 2.4-m deep) weighing lysimeters (0.05 mm accuracy, one in the center of each field); 20 neutron probe access tubes (readings at 0.20-m depth increments from 0.10 m to 2.30 m); 48 CS655 soil water sensors (8 vertical arrays of sensors installed horizontally at depths of 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30 m in each array), and a COsmic-ray Soil Moisture Observing System (COSMOS) positioned at the weighing lysimeter in the center of one field. Drip laterals were spaced 1.52 m apart and buried at 0.30 m depth, with emitters on 0.30 m spacing along each lateral. Although the weighing lysimeters reported the total amounts of drip irrigation and precipitation accurately, they yielded no information on where in the root zone the water was. The COSMOS unit was sensitive to precipitation, but not very sensitive to water applied through the SDI system. Water contents from the COSMOS unit were biased to larger values by green growing vegetation. Total water content was difficult to ascertain with COSMOS due to uncertainty about the effective depth of measurement. Positioning of the neutron probe and CS655 sensors relative to the drip laterals influenced readings from each of these sensor types. Each sensor type reports unique and valuable information, but no one sensor type serves all purposes.