Despite covering an area larger than any irrigated crop in the United States and significantly contributing to outdoor water use in arid and semi-arid regions, irrigated lawns remain a major uncertainty in municipal and regional water budgets. Urban lawns are relatively small in size and have highly heterogenous urban surroundings, complicating methods of measuring and modeling evapotranspiration (ET). Using a calibrated static chamber method, we measured growing season ET from residential lawns in 6 cities representing a range of climatic conditions (Baltimore, Boston, Los Angeles, Miami, Minneapolis-St. Paul, and Phoenix). Overall, we measured ET from 149 irrigated lawns located in residential yards classified as fertilized, unfertilized, certified wildlife habitat, or designed to conserve or retain water (containing a xeriscape garden or water retention feature).
Results/Conclusions
Midday ET from irrigated residential lawns varied from 0.1±0.1 mm/h in Boston to 0.4±0.1 mm/h in Phoenix, with higher ET in arid and semi-arid than mesic cities. There was no relationship between ET and yard design/management, turfgrass species/height, or soil moisture variations, suggesting that all studied yards had ample soil moisture. Indeed, soil moisture measured in the top 10 cm was almost always higher than non-turfgrass landcover within the same yard and nearby unirrigated and natural landscapes. The best predictors of ET within and across the cities were the intensity of incoming solar radiation and atmospheric vapor pressure deficit. Overall, lawn ET was relatively well predicted across all cities with a Penman-Monteith equation of reference ET (ETo) under unlimited soil moisture conditions. The comparison of measured ET to ETo calculated for each yard revealed very close correspondences for all cities except Los Angeles and Phoenix, where lawn ET exceeded ETo by 0.5-1 mm/h. Our results highlight the high water vapor fluxes of irrigated residential lawns across the United States, as well as the need to improve the accuracy of ETo models for semi-arid and arid urban locations.