Despite extensive studies on hydrological responses to forest cover change in small watersheds, the hydrological responses to forest change and associated mechanisms across multiple spatial scales have not been fully understood. This review thus examined about 312 watersheds worldwide to provide a generalized framework to evaluate hydrological responses to forest cover change and to identify the contribution of spatial scale, climate, forest type and hydrological regime in determining the intensity of forest change related hydrological responses in small (<1000 km 2) and large watersheds (≥1000 km 2). Key findings include: 1) the increase in annual runoff associated with forest cover loss is statistically significant at multiple spatial scales whereas the effect of forest cover gain is statistically inconsistent; 2) the sensitivity of annual runoff to forest cover change tends to attenuate as watershed size increases only in large watersheds; 3) annual runoff is more sensitive to forest cover change in water-limited watersheds than in energy-limited watersheds across all spatial scales; and 4) small mixed forest-dominated watersheds or large snow-dominated watersheds are more hydrologically resilient to forest cover change. These findings improve the understanding of hydrological response to forest cover change at different spatial scales and provide a scientific underpinning to future watershed management in the context of climate change and increasing anthropogenic disturbances.

Abstract:Extensive studies on hydrological responses to forest change have been published for centuries, yet partitioning the hydrological effects of forest change, climate variability and other factors in a large watershed remains a challenge. In this study, we developed a single watershed approach combining the modified double mass curve (MDMC) and the time series multivariate autoregressive integrated moving average model (ARIMAX) to separate the impact of forest change, climate variability and other factors on dry season runoff variation in two large watersheds in China. The Zagunao watershed was examined for the deforestation effect, while the Meijiang watershed was examined to study the hydrological impact of reforestation. The key findings are: (1) both deforestation and reforestation led to significant reductions in dry season runoff, while climate variability yielded positive effects in the studied watersheds; (2) the hydrological response to forest change varied over time due to changes in soil infiltration and evapotranspiration after vegetation regeneration; (3) changes of subalpine natural forests produced greater impact on dry season runoff than alteration of planted forests. These findings are beneficial to water resource and forest management under climate change and highlight a better planning of forest operations and management incorporated trade-off between carbon and water in different forests.

Forest disturbance thresholds, defined as those at or above which significant hydrological impacts are caused, are important guides to support forest and watershed management decisions for protecting hydrological functions and minimizing negative environmental impacts. Our literature review suggests that despite their significance, the research on this topic is surprisingly limited (<20 publications), where the paired watershed experiments (PWEs) primarily designed for detecting hydrological responses to forest cover change at the small watersheds were used to derive the thresholds. However, the widely used thresholds (e.g., 20%) based on the PWEs were identified from visual interpretation rather than determined from hydrological response curves, suffering from methodological shortcomings, and thus, may lack reliability. To advance this topic, we provided a robust technique (the modified double mass curve, MDMC) for quantitatively determining forest disturbance thresholds on annual mean flow as it allows the development of a hydrological response curve between cumulative hydrological effects and forest disturbance over time at the watershed scale. We applied this robust technique in eight large watersheds in British Columbia, Canada, and found that the forest disturbance thresholds ranged from 12 to 25%. We highly recommend that the widely used forest disturbance thresholds must be reexamined, and more studies are needed with rigorous methods and in consideration of other hydrological variables in forested watersheds.

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