Integrating Landscape Metrics and Hydrologic Modeling to Assess the Impact of Natural Disturbances on Ecohydrological Processes in the Chenyulan Watershed, Taiwan

Abstract

The Chenyulan watershed, located in the central mountain area of Taiwan, has been suffering from earthquakes, typhoons, and heavy rainfalls in recent decades. These sequential natural disturbances have a cumulative impact on the watershed, leading to more fragile and fragmented land cover and loss of capacity of soil water conservation. In this study, the Soil and Water Assessment Tool (SWAT) and a landscape metrics tool (FRAGSTATS) were used to assess the direct impact (e.g., by annual rainfall) and indirect impact (e.g., by landscape configuration and composition) of natural disturbances on the ecohydrological processes of the Chenyulan watershed. Six SPOT satellite images from 2008 to 2013 were analyzed by using the nearest feature line embedding (NFLE) approach and reclassified into six land cover types: forest, cultivated land, grassland, river, landslide, and built-up. Forest was found to have the largest patch size, indicating that it is more resilient to disturbances, while agricultural land tended to expand from the river side toward the hill. Two land cover change scenarios were compared in the SWAT model. The results showed that there was no significant difference in simulated streamflow during 2004–2015 and sediment loading during 2004–2009; however, the model performed better for sediment loading during 2010–2015 with dynamic land cover change (coefficient of determination (R2) = 0.66, Nash-Sutcliffe efficiency coefficient (NSE) = 0.62, percent bias (PBIAS) = 10.5%, root mean square error observation standard deviation ratio (RSR) = 0.62) than with constant land cover (R2 = 0.61, NSE = 0.54, PBIAS = −17.3%, RSR = 0.68), indicating that long-term land cover change should be considered in hydrologic modeling. Changes in landslides during 2008–2013 were found to significantly affect ecohydrological processes, especially after 2011. In general, annual precipitation plays a dominant role, and landscape composition had by far the strongest influence on water yield and sediment yield compared to landscape configuration. The results can be useful for understanding the effects of land cover change on ecohydrological processes in the Chenyulan watershed and the potential impact of ecohydrological changes on the environment and public health.

Increased Nonstationarity of Stormflow Threshold Behaviors in a Forested Watershed Due to Abrupt Earthquake Disturbance

“…The abrupt flow process was mainly affected by subsurface stormflow and Horton overland flow generations at the foot of slope subjected to storm size and intensity rather than antecedent soil moisture (Farrick and Branfireun, 2014;Zhang et al, 2021a). Figure 6g shows the large expansion of VSA related to landslides which generally destroy the crown canopy, litter layer, and root-soil system in hillslopes (Chiang et al, 2019;Zhang et al, 2021b). These destructive processes alter the original landscape, with the formation of bare rock with steep slopes and accumulations of loose materials.…”

“…The unstable disturbances from endogenous (earthquake) and exogenous (rainstorms and concomitant hydro-geohazards) origins remarkably increase the uncertainty in the assessment of the hydrological regime from disturbance to recovery and flood risk management (Seidl et al, 2017). Previous studies have suffered from over-calibrated hydrological models (Chiang et al, 2019;Maina and Siirila-Woodburn, 2019;Tunas et al, 2020) and lack of understanding of runoff generation mechanisms in exploiting the effects of natural disturbance events on streamflow response. The efficient identification of nonlinear hydrologic behaviors in an earthquake-affected watershed as well as the understanding of post-earthquake long-term dynamics of hydrologic thresholds patterns is still urgently needed.…”

Assessment of Sediment Transport Functions with the Modified SWAT-Twn Model for a Taiwanese Small Mountainous Watershed

“…The observed streamflow from 2004 to 2015 was used for model calibration and validation. The streamflow-related parameters for calibration were referred to the previous study [30]. The parameters include curve number (CN2), plant uptake compensation factor (EPCO), surface runoff lag time (SURLAG), baseflow alpha factor (ALPHA_BF), effective hydraulic conductivity in main channel alluvium (CH_K2), and Manning's "n" value for the main channel (CH_N2).…”

“…Soil data was collected from [42]. Approximately 82% of the total area has not been surveyed, thus it was assumed to be darkish colluvial soil in this study [30]. The pale colluvial clay accounts for 12% of the study area.…”

Evaluation of Water Provision Ecosystem Services Associated with Land Use/Cover and Climate Variability in the Winike Watershed, Omo Gibe Basin of Ethiopia

“…Water yield is defined as the maintenance of water by ecosystems within a certain period (Xu et al 2016 ). Other authors also defined water yield as the sum of surface runoff from the landscape (Chiang et al 2019 ; Tallis et al 2011 ). The relative amount of water in a given landscape affects the quality of the environment by either increasing or decreasing land productivity (Shoyama and Yamagata 2014 ; Srichaichana et al 2019 ).…”