Evapotranspiration and Growth Based Models to Estimate Irrigation of Container-Grown Korean Spiraea (Spiraea fritschiana 'Wilma') Under Four Irrigation Regimes

Irrigation scheduling and water use are important issues facing the nursery industry as concerns about water use regulation, fresh water supplies, and runoff quality are forcing growers to develop water conserving irrigation programs.  Scheduling irrigation of container-grown woody ornamentals using reference potential evapotranspiration (ET_P) and canopy characteristics or crop coefficients to estimate actual evapotranspiration (ET_A) can conserve water compared to fixed irrigation rates.  The difference in substrate volumetric moisture content 1 hr and 24 h after irrigation was used to calculate ET_A of 10.2 L container-grown Spiraea fritschiana Schneid. ‘Wilma’ plants under 4 irrigation treatments.  Irrigation treatments were 1. a control irrigation rate of 19 mm-ha applied per irrigation application; 2. irrigation scheduled to replace 100% daily water use (DWU) per application (100DWU); 3. irrigation alternating every other application with 100% replacement of DWU and 75% DWU (100-75); and 4. an irrigation application replacing 100% DWU followed by two applications replacing 75% DWU (100-75-75).  Irrigation applications were separated by at least 24 hours.  Relationships of ET_A to reference potential evapotranspiration (ET_P) and growth index (GI) were investigated using regression analysis.  The dependent variable ET_A was log transformed for all treatments.  Relationships for the control, 100DWU, 100-75, and 100-75-75 treatments included the independent variables ET_P, GI, and GI² (R² = 0.704); ET_P and GI (R² = 0.479); ET_P, GI, and GI² (R² = 0.438); and ET_P (R² = 0.424), respectively.  Estimated crop coefficients of the control, 100DWU, 100-75, and 100-75-75 treatments fluctuated during the growing season and ranged from 1.7 to 3.5; 1.7 to 4.3; 2.0 to 5.4; and 1.6 to 6.7, respectively.  Relationships of ET_A to ET_P and GI suggest that with further research and validation working models for irrigation scheduling of container-grown Spiraea fritschiana ‘Wilma’ can be developed.