Submitted to: Irrigation Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/24/2008
Publication Date: 3/3/2009
Citation: Jabro, J.D., Evans, R.G., Kim, Y., Iversen, W.M. 2009. Estimating in situ soil-water retention and field water capacity measurements in two contrasting soil textures. Irrigation Science. 27:223-229.
Interpretive Summary: Optimal irrigation management practices for crops require measurement of soil water retention data in the field to assess both the amount and timing of irrigation. The in-situ soil water retention curves from simultaneous soil ' and volumetric ' measurements obtained from the WM and TDR sensors were developed for both sandy loam and clay loam soils. The Campbell (Campbell, 1974) and Gardner (Gardner, 1958) equations provided the best fit for the soil water retention curves with R2 = 0.97 and 0.96 for sandy loam and clay loam soils, respectively. The changes of soil ' with time following cessation of infiltration were well described by 3-parameter sigmoid models with R2 = 0.997 and 0.936 for sandy loam and clay loam soils respectively. Based on these relationships, the tFWC were reached at approximately 50 and 450 hrs following cessation of infiltration and soil 'FWC values at these two elapsed times were approximately 18 and 27 kPa for sandy loam and clay loam soils, respectively. Using soil water retention curves, the corresponding 'FWC values at 50 and 450 hours were approximately 0.228 and 0.344 m3 m-3 for sandy loam (Nesson) and clay loam (EARC) soils, respectively. The estimated 'FWC values were within the range of the measured 'FWC values obtained from the NP probe and gravimetric methods. These results indicated that WM and TDR sensors provided accurate in-situ soil water retention data that can be used in agricultural and environmental applications including irrigation management and scheduling.
Technical Abstract: Knowledge of the in-situ field water capacity (FWC) and the soil-water retention curve for soils is important for effective irrigation management and scheduling. The primary objective of this study was to estimate the in-situ FWC from the soil-water retention curve developed from water content, ' and water potential, ' data collected in the field using soil moisture sensors in two contrasting-textured soils. The two soils are Lihen sandy loam and Savage clay loam. Six 117 cm * 117 cm metal frames, 30 cm in height were inserted 5- to 10 cm into soil to prevent lateral movement. The frames were spaced at approximately 40 m intervals on a 200 m transect to account for soil variability across the field. Two Time Domain Reflectrometry (TDR) sensors were installed in the center of the frame and two Watermark (WM) sensors were installed in the SE corner at 15 and 30 cm depths to continuously monitor soil ' and ', respectively. A neutron probe (NP) access tube was installed in the NE corner of each frame to measure soil ' used for TDR calibration. The NP readings were taken every 6 hours in sandy loam and daily in clay loam soil. The soil inside each frame was saturated by applying approximately 18-20 cm of water intermittently to saturate the soil to a 50 - 60 cm depth. Frames were then covered with plastic tarps to prevent any evaporation from the soil. Nine soil cores were taken at 0 to 10 cm, 10 to 20 cm and 20 to 30 cm depths from center of each framed area at both locations to determine gravimetric '. The Campbell and Gardner equations best fit the soil water retention curves for sandy loam and clay loam soils, respectively. Based on the relationship between soil ' and elapsed time following cessation of infiltration, the field capacity times, tFWC were reached at approximately 50 and 450 hrs for sandy loam and clay loam soils, respectively. Using soil-water retention curves, ' values at field water capacity, 'FWC were approximately 0.228 and 0.344 m3 m-3 for sandy loam and clay loam soils, respectively. The estimated 'FWC values were within the range of the measured 'FWC values from the NP and gravimetric methods. The TDR and WM sensors provided accurate in-situ soil water retention data from simultaneous soil ' and ' measurements that can be used in soil-water processes, irrigation scheduling, modeling and chemical transport.