How will land use affect air temperature in the surface boundary layer? Lessons learned from a comparative study on the energy balance of an oak savanna and annual grassland in California, USA

Abstract

A B S T R A C TWe investigated the effect of land use on differences in air temperature. We based our analysis on a decade of weather and energy flux measurements, collected over two contrasting landscapes, an oak savanna and an annual grassland, growing under the same climate conditions. Over the decade, the daily-averaged, potential air temperature above the aerodynamically rougher and optically darker oak savanna was 0.58C warmer than that above the aerodynamically smoother and optically brighter annual grassland. However, air temperature differences were seasonal. Smallest differences in potential air temperature occurred towards the end of spring, when much of the soil moisture reservoir was depleted. Largest differences in potential air temperature occurred during the winter rain season when the grass was green and transpiring and when the trees were senescent or deciduous. To understand the effect of land use on the local climate, we examined the concomitant changes in net radiation, sensible and latent heat exchange, the aerodynamic roughness (R a ), the surface resistance to water transfer (R s ), aerodynamic surface temperature and the growth of the planetary boundary layer, with measurements and model computations. Overall, these biophysical variables provide us with mechanistic information to diagnose and predict how changes in air temperature will follow changes in land use or management. In conclusion, land use change is responsible for having a marked impact on the local climate of a region. At the local level, the change in the surface energy balance, towards a darker and rougher surface, will produce an additive increment to climate warming induced by a greater greenhouse gas burden in the atmosphere.

Land management and land-cover change have impacts of similar magnitude on surface temperature

“…Mean and upper limit temperature responses to LCC and LMC were similar (Fig. 1b) and around 2 K at the site-level, with a maximum of 6 K. Because LCC was used as a benchmark for LMC and our findings for LCC are in agreement with previous observations regarding LCC in the tropics 3,7 , Mediterranean 19 and the boreal zone 6 our study lends support to the hypothesis that LMC other than irrigation 10,11 also has substantial biophysical impacts on the land surface.…”

“…4c, p = 0.33). Larger ranges and loss of correlation illustrate the complexity of the effects of land use change on atmospheric temperature 19 , or the plausibility of the current PBL model, which does not capture any cloud or advective processes 28 .…”

Warming and Elevated CO2 Have Opposing Influences on Transpiration. Which is more Important?

“…A1a was set as a constant for general vegetation types. We used daily average values measured during mid-summer at a woodland and grassland site in California [80] The canopy conductance, g c , in Eq. A1 was either calculated explicitly from stomatal conductance in relation to photosynthetic carbon gain, as described below, or set to specific prescribed values for some generic illustrations to: …”

“…This is calculated for typical forests (a, c) and grasslands (b, d) that are either stressed or wellwatered, with corresponding differences in canopy conductances. We assumed aerodynamic conductances of 3.33 cm s −1 for forests and 1.54 cm s −1 for grasslands [80] and canopy resistances of 2 and 0.5 cm s −1 for unstressed and stressed plants, respectively. Net radiation was kept at 400 μmol (quanta) m −2 s −1 for all simulations, and the daytime minus night-time temperature difference was 10°C closure, forests can also exert greater control over their transpiration rates than grasslands.…”

Quantifying the contribution of land use change to surface temperature in the lower reaches of the Yangtze River

“…Agriculture often leads to cooling temperature in different patterns, and the cooling effect can usually be magnified when it comes to irrigation (Campra et al, 2008;Kueppers et al, 2007;Lobell et al, 2006;Zhang et al, 2011). For this reason, analyzing different types of land use plays an important role not only in evaluating climate change on different spatial scales (Alkama and Cescatti, 2016;Baldocchi and Ma, 2013;Huang et al, 2008;Wang et al, 2010;Hari et al, 2016), but also in improving the predictive capacity of models (Huang et al, 2015;Niu et al, 2011;Zhang et al, 2015). Although there have been many studies concentrating on LULCC, they rarely compare the differences in the mechanisms behind the land-atmosphere interaction with different types of land use.…”