Positive and neutral effects of forest cover on dry‐season stream flow in Costa Rica identified from Bayesian regression models with informative prior distributions

Abstract

The influence of forest cover on dry-season stream flow in the humid tropics has been of enduring interest to eco-hydrologists and policy makers. We used data from eight nested gauging stations in the Terraba river basin (4774 km 2 ) of Costa Rica to demonstrate how a Bayesian regression approach could be applied to test hypotheses on "infiltration-evapotranspiration trade-offs", between the "sponge effect" and evapotranspiration demands of forests on dry-season flows. We summarized three competing hypotheses as informative prior distributions for regression slopes as 1) positive, 2) zero, and 3) negative effect of forest cover, and combined them with dry-season flow data (response), and annual rainfall and per cent forest cover (as covariates) to obtain posterior estimates. The time series included a deforestation phase until the 1980s and subsequent forest recovery from the 1990s. The sign, magnitude, and uncertainty of posterior slopes showed that forest cover effects on dry-season flow were generally positive or neutral, influenced by both forest loss and recovery. Forest cover effect on dry-season flow for the entire basin was strongly positive. Four sub-catchments showed weakly positive and two showed neutral effects, and only one indicated a weakly negative effect. Mean effect size of forest cover diminished with increase in basin size. Our results predict a gain of 3.3 mm in dry-season flow for every 1% increase of forest cover, corresponding to 48 km 2 of basin area. Enhanced infiltration and wet-season groundwater recharge dominating evapotranspiration can be attributed partly to deep soils and cloud forests. We demonstrate how informative prior distributions can represent different hypotheses in Bayesian regression models offers advantages in investigating complex hydrological processes. The rigorous evidence for positive and neutral effects of forest cover on dry-season flow, reported by our study, potentially contributes to ecohydrology-based management of tropical landscapes undergoing rapid change.

Forest regeneration can positively contribute to local hydrological ecosystem services: Implications for forest landscape restoration

“…Some studies have started to explore the ecohydrological effects of tropical reforestation using streamflow data and hydrological models (Krishnaswamy et al., 2018; Lacombe et al., 2016; Ribolzi et al., 2018) but empirical evidence on how this trade‐off plays out in practice is very limited because the data are difficult to collect. Field studies exploring the effects of reforestation in terms of both overland flow and evapotranspiration are rare, and very few studies have quantified the net effect of reforestation on net recharge (i.e.…”

Water budget and run‐off response of a tropical multispecies “reforest” and effects of typhoon disturbance

“…Nevertheless, the fact that streamflow at Manobo became perennial in 1998, a very dry El Niño year that was preceded by three wetter years (Figure S6b), suggests that by 1998 the moisture infiltration and storage capacity of the soil may have increased sufficiently (cf. Y. Cheng et al, ; Hou et al, ; Krishnaswamy et al, ; J. Zhang, Bruijnzeel, Quiñones, et al, ).…”

“…Y. Cheng et al, 2017;Hou et al, 2018;Krishnaswamy et al, 2018;J. Zhang, Bruijnzeel, Quiñones, et al, 2018).…”

“…This is usually referred to as the “infiltration‐trade‐off” hypothesis (Bruijnzeel, ). Several examples of gradually improved streamflow regimes (reduced peaks, increased or longer dry‐season flows) after reforesting severely degraded land (Choi & Kim, ; Hou et al, ; Krishnaswamy et al, ; Krishnaswamy, Kelkar, & Birkel, ; Zhou et al, ) have suggested the validity of the trade‐off mechanism (cf. Y. Cheng, Ogden, & Zhu, ).…”

India’s Commitments to Increase Tree and Forest Cover: Consequences for Water Supply and Agriculture Production within the Central Indian Highlands

“…The infiltration-evapotranspiration trade-off hypothesis provides a framework for understanding the possible alteration of the hydrological cycle from reforestation and afforestation [14][15][16][17]. As compared to other land cover, forests have higher rates of evapotranspiration (ET) but also have higher infiltration and groundwater recharge [15,18].…”