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[1] This study employs a dual-isotope (d 18 O and dD), three-end-member linear mixing model to semiquantitatively assess vapor contributions of advection, transpiration, and evaporation to precipitation, and to compare the extent of moisture recycling (including transpiration and evaporation) among areas of various topographic types in Taiwan, an island located in the western Pacific. The three topographic types examined are the mountainous regions, foothill regions (as represented by two reservoir stations), and coastal plains. Results indicate that the moisture fractions from advection and evapotranspiration in mountain precipitations are about 63% and 37%, respectively, and those in precipitation of plain and foothill regions are about 69% and 31%, respectively. Moreover, transpiration accounts for most of the recycling moisture while evaporation offers minor contribution. Since the transpiration fraction of recycling moisture is higher in forest mountain area than in plain/foothill region, topographic type is the major factor affecting the extent of transpiration. Nevertheless, the two reservoirs examined in this study do not offer significant contribution of recycling moisture to local precipitation. In addition to topographic type, temperature and rainfall may be two other factors controlling the extent of transpiration; transpiration would be promoted in greater rainfall regions but reduced in higher temperature areas. Additionally, about 25% of precipitation in plain/foothill region is of relocated moisture from evapotranspiration induced from forest mountains. It should be noted that this isotope-based approach has its limitations and should be applied with caution.

Abstract. Nitrate export in small subtropical watersheds is rarely observed and the estimation of individual land use nitrate yield from a mixed combination within catchments has scarcely been studied. In this study the nitrate concentrations at 16 nested catchments in the Chi-Chia-Wan watershed in Central Taiwan were measured during [2007][2008]. A 3-layer TOPMODEL was applied to estimate daily discharge for ungauged sub-catchments. The observed nitrate concentrations and the simulated discharges were used for nitrate flux estimations through four flux methods. Meanwhile, a new deconvolution computation was developed to resolve the nitrate yield of each land use from within the mixed combinations.The results showed that the observed mean NO 3 -N concentration in relatively pristine catchments was approximately 0.145 ± 0.103 mg l −1 , which is comparable with other forestry catchments around the world. However, the higher rainfall/runoff, substantial N deposition, and other nitrogen sources resulted in significantly higher annual export of approximately 238-1018 kg-N km −2 yr −1 . Our deconvolution computation showed that the background yield of natural forestry was ∼351 ±62 kg-N km −2 yr −1 . On the other hand, the extremely high nitrate yield of active farmland was ∼308, 170 ± 19 241 kg-N km −2 yr −1 due to overfertilization. The deconvolution computation technique is capable of tracing the mixed signals at the outlet back to the nitrate productions from varied land use patterns. It advances the application of river monitoring network. The typical values of nitrate yields can serve as a guideline for land management. Comparing the nitrogen input and output, we found some nitrogen missing in the cycling which may indicate certain removal processes and we therefore suggest further study to be carried out to fully understand nitrogen cycling in subtropics.

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Submarine groundwater discharge (SGD) is the submarine seepage of all fluids from coastal sediments into the overlying coastal seas. It has been well documented that the SGD may contribute a great deal of allochthonous nutrients to the coastlines. It is, however, less known how much carbon enters the ocean via the SGD. Nutrients (NO3, NO2, NH4, PO4, SiO2), alkalinity and dissolved inorganic carbon (DIC) in the submarine groundwater were measured at 20 locations around Taiwan for the first time. The total N/P/Si yields from the SGD in Taiwan are respectively 3.28 ± 2.3 × 104, 2.6 ± 1.8 × 102 and 1.89 ± 1.33 × 104 mol/km2/a, compared with 9.5 ± 6.7 × 105 mol/km2/a for alkalinity and 8.8 ± 6.2 × 105 mol/km2/a for DIC. To compare with literature data, yields for the major estuary across the Taiwan Strait (Jiulong River) are comparable except for P which is extremely low. Primary production supported by these nutrient outflows is insufficient to compensate the DIC supplied by the SGD. As a result, the SGD helps making the coastal waters in Taiwan and Jiulong River heterotrophic.