Alterations in ecosystem water cycle associated with land- use changes under different precipitation regimes

Vegetation and soil can emit in water vapor by evapotranspiration (ET) in quantities equivalent to the entire annual local precipitation (P). This can greatly impact the local hydrology, leaving a small residual [P-ET], termed ecosystem water yield (WYe) in the fresh water that is available for human consumption. The general objective of this study is to examine the interactions between land-use change (from sparse shrub land to pine forest) and precipitation regime (from humid Mediterranean to semi-arid conditions) on WYe, and to identify the mechanisms underlying these interactions. We hypothesized that forestation increases ET and therefore reduces WYe by increasing the transpiring leaf area and root depth, but this effect diminishes with decreasing precipitation. We used a new custom-built mobile laboratory that was deployed on a campaign basis (about two weeks per site, repeated along the seasonal cycle), which allowed us to measure ecosystem-scale ET together with carbon, energy and ozone fluxes, as well as meteorological parameters. Measurements were carried out from 2012-2015 in three paired sites of Pinus halepensis forests and adjacent non-forest ecosystems along the rainfall gradient in Israel, ranging from 755 to 285 mm of annual precipitation. The extension of the campaign results to annual ET is based on multiple-regression analyses with continuous meteorological records from local meteorological stations, using the Flux-Meteorological-Algorithm. The physiological parameters, such as transpiration (T) and stomatal conductance (gs) were estimated using sap flow (SF) sensors and gas-exchange leaf chamber respectively. In dry conditions, WYe was small and the forest impact was limited, reducing WYe from 70 mm in the non-forest ecosystem to 12 mm in the forest ecosystem (-58 mm). Under wetter conditions, the forestation effect was greater, reducing WYe from 344 to 144 mm (-200 mm). This was mostly due to the greater increase in ET from the dry to the wetter forest (from 267 to 583 mm or x2.2), compared to the non-forest areas (from 216 to 383 mm, or x1.8). The phenology of ET also changed, with the forest peak ET shifting from early March in the dry site to late May in the humid sites and with the non-forest peak ET shifting from late February in the dry site to late March in the humid site. The seasonal campaigns show changes in the diurnal pattern of measured water fluxes at all scales (ET, T and gs) in response to changes in climatic conditions, thereby indicating differential environmental effects (e.g., global radiation and water-vapor pressure deficit) and physiological control (e.g., gs and SF). We concluded that afforestation is always associated with a loss in Wye, due to increased ET, but that the afforestation impact on WYe is non-linear, with diminishing effects in water-limited environments and approaching a higher and constant effect (with ET reaching a maximum) in wet environments. In this work, we 1) demonstrated a new measurements approach based on the mobile lab; 2) developed and validated the Flux-Meteorological-Algorithm; 3) based on 1 & 2, provided the first quantitative results of the effects of land-use changes on the local hydrology under increasingly drying conditions, as predicted for our region. This study indicates that, while the hydrological impact of forestation in humid Mediterranean conditions is important to consider, the benefits of forestation in increasingly drying conditions (soil protection, surface cooling, wood production and so forth) may outweigh the diminishing hydrological impact. Table of