Tuesday, November 14, 2006

Irrigation Coefficients for Mixed Urban Landscapes.

James Thomas, Soil & Crop Sci.Dept.,TAMU, Rm 548 Heep Center, College Station, TX 77843-2474, United States of America, Roger Havlak, Texas A&M University, 3355 Cherry Ridge, Suite 212, San Antonio, TX 78230, United States of America, David Chalmers, Soil & Crop Sciences, Texas A&M Univ. 233-a Heep Center, College Station, TX 77843-2474, United States of America, Richard White, Soil & Crop Sciences Dept, Texas A&M University, College Station, TX 77843-2474, United States of America, and Wayne A. Mackay, Texas A&M University Dallas, Agricultural Research and Extension Center, 17360 Coit Road, Dallas, TX 75252, United States of America.

Landscape irrigation programs are often based on potential evapotranspiration (ETo) despite lack of data on actual evapotranspiration (ETa) of landscapes containing multiple plant species.  While irrigation coefficients have been determined for numerous turf species, little in situ water use information exists for trees, shrubs, or other ornamental plants.
The objective of this study was to determine the landscape coefficient (KL) for a mixed urban landscape in South Texas.  Soil moisture sensors were installed at 64 locations at depths of 0 to 20, 20 to 40, and 40 to 60 cm to record volumetric soil water content at 30-min intervals.  Changes in volumetric water content between midnight of successive days for each week over a 2-yr period were used to determine weekly ETa. ETo was calculated using data from an on site weather station.  The weekly KL was calculated from the ratio of ETa to ETo.  The majority of weekly KL values ranged from 0.2 to 1.0 and were influenced by soil moisture levels and season of the year.  KL values were correlated to soil moisture content when soil moisture was less than 0.23 cm3 cm-3.  When soil moisture was greater than 0.23 cm3 cm-3, ETa was correlated to ETo.  These relationships indicate that atmospheric demand regulated water loss when soil moisture was plentiful but soil moisture became the primary regulating factor as soil moisture was depleted below a critical level.  Thus, irrigating a mature urban landscape to sustain high soil moisture content maximizes ETa and wastes municipal water supplies.
Comparison of the measured KL values to those predicted using the procedure of Costello et al., 2000 showed that for the conditions of this study the Costello method overestimated the actual KL.  Additional studies using different vegetation types and different climatic conditions are needed to further refine the relationships defining KL.

Handout (.pdf format, 96.0 kb)