Substantial Nighttime Evapotranspiration and Soil Evaporation in Agrostis Stolonifera.

Turf water requirements are often estimated by using the Penman-Monteith model.  However, the Penman-Monteith assumes no nighttime transpiration (T).  Nighttime T has been shown to represent a significant amount of water loss from various plant canopies.  If nighttime T ocurs in turfgrass systems it is likely that under some conditions they are under deficit irrigation.  In this study, evapotranspiration (ET) during the light and dark in response to vapor pressure deficit (VPD) was investigated in growth chambers.  Evapotranspiration from potted Agrostis stolonifera taken from field cores was measured continuously using automated balances.  All plants were subjected to various VPD treatments during the light and dark periods.  To partition the sources of ET, a set of pots was treated with an herbicide so that transpirational water loss would be excluded while retaining a physical barrier of resistance.  To calculate T, soil evaporation from herbicide-treated cores was subtracted from ET of living grass.  Nighttime ET ranged from about 0.05 mm h^-1 at VPD’s below 0.5 kPa to 0.35 mm h^-1 at VPD’s above 3.5 kPa and represented about 37-38% of daily ET across VPD values.  Nighttime T represented about 30% of daily T.  Soil evaporation was a larger component of ET than T in both the day and the night ranging from about 50-72% of ET.  However, there is likely an overestimation of E in our system.  Nevetheless, these results indicate that proper water management of soils may have a significant impact for reducing ET.  This applies particularly to locations where VPD may be high during the night.  Additionally, substantial nighttime T indicates that on some days during the season turfgrasses may be subjected to deficit irrigation.