Evapotranspiration of urban lawns in a semi-arid environment: An in situ evaluation of microclimatic conditions and watering recommendations

- The landscape coefficient method has not been verified in situ in urban settings.
- Lawns located in park-like environments may be strongly affected by heat advection.
- Shading reduces turfgrass water use, in particular during extremely dry weather.
- Inaccurate accounting for shading may lead to overirrigation.

As many regions worldwide are increasingly affected by water scarcity caused by unprecedented climate change and population growth, current practices of landscape irrigation need to be evaluated to develop scientifically based water-saving recommendations. We compared widely used evapotranspiration (ET) based guidelines of turfgrass watering requirements with ET from 8 irrigated turfgrass lawns (4 unshaded and 4 shaded by various degree) measured in situ by portable chambers in the Los Angeles Metropolitan area. ET from unshaded lawns was 40% higher than recommended irrigation inputs and ET from shaded lawns was mostly within recommended ranges. We evaluated the microclimate coefficient (kmc), a factor used to adjust ET from reference surfaces to local microclimatic conditions. While in situ kmc was mostly within the ranges recommended for California, (1) kmc ranges for particular landscape types deviated from observations, (2) minimum and maximum kmc were beyond recommended ranges, and (3) kmc had clear seasonal changes overlooked by current methodology. We propose a method to quantitatively estimate the effect of shade on kmc and improved categories and coefficients for applying the landscape coefficient method under realistic urban conditions.