More trees, more rain? The unexpected role of forest and aquifers on the global water cycle

Abstract. Forests’ influence on rainfall has been debated since antiquity, with historical observations suggesting that deforestation reduces precipitation. While early scientists believed that forests attract rain, later research provided conflicting views, and modern climate models remain inconclusive on forests’ role in regional precipitation at continental scale. A difficulty of global climate model is the need to integrate processes associated to multiple scientific field, like meteorology, hydrology and forest ecology. As a result, the numerous constitutive equations, the complexity of numerical codes and the variability of natural situations make difficult to provide a quantitative water cycle model controlled by a small number of key parameters. Assuming water mass conservation between ocean, atmosphere, ecosphere and continental aquifers, we revisit this problem and we build a simplified physical scheme of water transfer across these reservoirs. Assembling a limited number of constitutive equations into a numerical code, we propose a parsimony model able to simulate rainfall distribution patterns. Starting from initially different vegetation conditions (sparse and dense), we identify the parameters modulating steady state continental precipitation for both situations. First, we show that sparse vegetation patterns do not lead to uniform and large continental precipitation. Rather, continental moisture is spatially limited by atmospheric dispersion and wind, thus providing a decay of precipitation as a function of coastal distance as observed in most cases worldwide. Second, our model predicts that large and widespread continental precipitation results from the combination of dense vegetation patterns and deep aquifers low hydraulic conductivity. This ecological/hydrological interaction is due to the slowness of horizontal water flow, allowing underground reservoir level to increase enough for reaching plant roots, inducing in turn vegetation evapotranspiration and atmospheric moisture. Overall, our model shed new light on the balance between atmospheric water vapor transport by wind and dispersion, vegetation evapotranspiration and underground water flow. Noticeably, the efficiency of the water cycle and its related precipitation distribution not only depends from atmospheric parameters but is also largely modulated by vegetation dynamics and underground aquifers behavior. We finally discuss how forest growth or destruction may alter continental precipitation at various time scales.