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This study is to develop an isotopic catchment-effect index (CEI) connecting the physiographic characteristics of stream catchments. A CEI, describing the extent of difference in stable water isotopic compositions (б values) between stream water and local precipitation at any given sampling site, can help in judging whether water resource management should be focused on upstream regions of streams or local hydrology issues. To establish the isotopic CEI, this study measured б values of stream water and derived б18O of local precipitation based on regional isotopic altitude gradient at montane catchments of various sizes. Results indicate that the CEI is strongly related to catchment physiographic characteristics, such as length of main stream, mean area, mean elevation, perimeter and slope. These characteristics are considered important indices of streamflow. Based on mathematical regression modeling describing the relationships between CEI and respective physiographic factors, CEI values can predict respective physiographic factors and vice versa. Moreover, according to the multiple equations derived in this study, catchments of larger size and steeper slope give elevated CEI values while greater stream length reduces the CEI's value. A greater CEI value indicates that local stream water is principally sourced from upstream reaches rather than contributions from local precipitation. In addition, CEI values are greater in winter than in summer resulting from monsoon effect. Consequently, this study establishes CEI as a useful descriptor of the physiographic characteristics of catchments. [ABSTRACT FROM AUTHOR]

Summary Water isotopes (δD, δ 18 O) as well as electrical conductivity were employed to clarify hydrological relationships among precipitation, stream water, and groundwater in the Douliou Hills (DLH) of Taiwan. They are also used to evaluate the importance of groundwater recharge sources in the proximal fan of Choshuichi alluvial plain (CSAP), which is connected to the eastern extreme of the DLH. Results indicate that the most important source of groundwater in the DLH comes from summer precipitation. In most cases the summer precipitation recharges stream water via base flow. Based on δ 18 O and EC values, groundwater in the proximal fan of the CSAP can be divided into groundwater from 5 wells nearby the DLH (near-DHL group) and groundwater from 6 wells nearby the Choshui Stream (near-CS group). Semi-quantitative results calculated by ternary end-member mixing analysis (EMMA) for δ 18 O and electrical conductivity values indicate that for the near-DLH group, precipitation in the CSAP contributes 40–50% of water to the proximal-fan groundwater while DLH groundwater donates 20–55% and Choshui Stream water, 0–29%. Precipitation in the CSAP is the most important recharge source. DLH groundwater’s contribution is more notable in deep groundwater and Choshui Stream water’s contribution decreases with distance from the Choshui Stream. For the near-CS group, anthropogenic pollution adds ions to the groundwater restricting the effectiveness of the ternary EMMA approach. This study provides new insights into groundwater recharge in the CSAP. It should be applicable to future studies of hill-and-plains’ water connectivity in other regions. [ABSTRACT FROM AUTHOR]

Summary: Environmental isotopes including stable hydrogen and oxygen, and radioactive tritium isotopes are employed to assess groundwater sources in the Taoyuan–Chungli Tableland (TCT), northwestern Taiwan. We evaluate the contribution to groundwater resources of copious ponds distributed throughout the TCT. Results indicate that both precipitation and pond water are the end sources of proximal shallow groundwater (P-SGw) in the TCT. Recharge of P-SGw to deep groundwater in the proximal-fan region (P-DGw) is obvious, because δ values of P-DGw are similar to those of P-SGw and no barrier restricts P-SGw entering P-DGw. However, the infiltration of precipitation or pond water to shallow groundwater in the mid- and distal-fan regions (MD-SGw) is restricted somewhere because of impermeable confining layers overlying the MD-SGw. In addition, apart from the upstream recharge of the P-SGw, MD-SGw overlying the deep groundwater in the mid/distal-fan regions (MD-DGw) contributes to MD-DGw through barrier-layer leakage between MD-SGw and MD-DGw. Overall, δ values indicate that pond-sourced moisture accounts for about 6% of precipitation in the studied tableland. This contribution fraction is larger than that in the western plains or reservoir catchments of Taiwan. The calculations also indicate that precipitation and pond water respectively contribute 51% and 49% to P-SGw. Among the 49% contribution from pond water to P-SGw, 46% is from direct infiltration and the remaining 3% is via the evaporation–precipitation process. The fractional results clearly indicate that pond water is a vital source of both local precipitation and groundwater. [ABSTRACT FROM AUTHOR]