Mixed landscape water demand and drainage in a subtropical-monsoonal climate varies with irrigation frequency and season

Abstract

We used drainage lysimeters to study water balance and plant growth of mixed (woody plant-turf) 
landscapes irrigated at three rates of root zone water depletion based on local reference evapotranspiration 
(ETO) in a subtropical-monsoonal climate: slow/dry (60%), intermediate (75%) and fast/wet(90%) of ETO. 
Planting design for each lysimeter was identical: a Magnolia virginiana tree imbedded in St. Augustine grass 
(Stenotaphrum secundatum) turfgrass bordered by two Viburnum odoratissimum shrubs grown as a hedge. 
Landscape monthly actual evapotranspiration (ETA) was calculated as the difference between inputs 
(irrigation and precipitation) and output (drainage). Magnolia (height, projected canopy area, and trunk 
diameter) and viburnum (height and canopy volume) growth was measured every three weeks. Total 
drainage over three years from the 0.6 depletion (driest) treatment was 20 and 45 cm for the dry and wet 
season, respectively, half that of the 0.9 depletion (wettest) treatment. The dry treatment had the lowest ETA 
both seasons, and average dry season Plant Factors (PF=ETA÷ETo) was about 15% lower than the wet 
season (0.76 versus 0.98). Plant growth was largely unaffected by irrigation-depletion treatments. For 
subtropical climates, dry season irrigation scheduling based on a mixed landscape PF of 0.75 is reasonable 
to maximize growth following transplanting. Once established, dry season irrigation schedules can be based 
on a 0.6 PF value to maintain performance while minimizing drainage. Irrigation of mixed landscapes during 
the monsoonal wet season is only needed during exceptional dry periods.