Landscape Coefficients for Single- and Mixed-species Landscapes

Abstract

Urban landscape irrigation is becoming increasingly important from a resource management point of view. Significant water use savings may be achieved if landscape irrigation is based on reference evapotranspiration (ET o ). This study measured landscape crop coefficients (K L ) for landscapes that are comprised of different vegetation types and irrigation water quality differences affecting K L . The K L was determined from the ratio of actual evapotranspiration to the ET o calculated from the modified Penman-Monteith equation. Irrigation quantity was based on 100% replacement of ET o . The K L values were determined for the following landscape vegetation on a fine sandy loam: St. Augustinegrass [ Stenotaphrum secundatum (Walt.) Kuntze.], a single shumard red oak ( Quercus shumardii Buckl.), St. Augustinegrass plus red oak, native grasses [ Muhlenbergia capillaries (Lam.) Trin. and Schizachyrium scoparium (Michx.) Nash], and native grasses plus Red Oak in both College Station (CS) and San Antonio (SA), TX. Soil was systematically placed into lysimeters containing a drainage system and soil moisture probes. Lysimeters (1136 L) were placed in-ground in a randomized complete block design with three blocks. Soil moisture measurements were made at 0- to 20-, 20- to 40-, and 40- to 60-cm depths. The K L was determined after a rainfall or irrigation event for periods of 2 to 5 days. During the combined growing seasons of 2007 and 2008, K L in SA increased from early, to mid, to late season. In CS, the K L was unaffected by plant treatment or season. The St. Augustinegrass treatment K L seasonally ranged from 0.45 to 0.62 in SA. In CS, soil sodium accumulation caused decreased K L . These results of K L for mixed-species landscapes on non-sodic sites trend toward seasonal values of 0.5 to 0.7 for irrigation decisions in southern Texas. Landscape coefficients can be used as a tool in irrigation decision-making, which could contribute to water savings in amenity landscapes.