Production of storm runoff in highly responsive catchments is not well understood. We report in these papers a comprehensive set of hydrometric and natural tracer data for rainfall, soil water, and streamflow for catchments in the Tawhai State Forest, Westland, New Zealand, which reveal some of the important runoff processes. The catchments are small (< 4 ha), with short (< 300 m) steep (average 34°) slopes and thin (< 1 m) permeable soils. Long‐term (1977–1980) weekly observations of oxygen 18, electrical conductivity, and chloride in the stream, groundwater, and rain in the main study catchment indicate that catchment outflow reflects a well‐mixed reservoir with a mean residence time of approximately 4 months. A preliminary storm hydrograph separation using oxygen 18 (for a storm hydrograph exceeded by only 22% of events since 1979) indicates that only 3% of storm runoff could be considered “new” (i.e., current storm) water. Rapid subsurface flow, such as macropore flow, of new water therefore cannot explain streamflow response in the study area. More detailed hydrograph separation studies on throughflow as well as streamflow are described in parts 2 (M. G. Sklash et. al., this issue) and 3 (M. G. Sklash et. al., unpublished manuscript, 1986).

Isotopic variation in storm rainfall is an important consideration in hydrograph separation using the mass balance approach but is rarely considered when determining the accuracy of old water estimates. Study of a small watershed on the South Island of New Zealand in which new water is a major component of the storm hydrograph shows that, in addition to the within‐storm isotopic variations themselves, rainfall weighting techniques may substantially influence estimates of old/new water as a function of both total runoff and total quick flow production. Two incremental approaches to rainfall weighting are presented. Results show that within‐storm incremental weighting is better than the standard weighting technique, which imposes a total storm rainfall value exogenously on the mass balance equation.

Abstract. Tritium measurements of streamwater draining the Toenepi catchment, a small dairy farming area in Waikato, New Zealand, have shown that the mean transit time of the water varies with the flow rate of the stream. Mean transit times through the catchment are 2-5 years during high baseflow conditions in winter, increasing to 30-40 years as baseflow decreases in summer, and then dramatically older water during drought conditions with mean transit time of more than 100 years. Older water is gained in the lower reaches of the stream, compared to younger water in the headwater catchment. The groundwater store supplying baseflow was estimated from the mean transit time and average baseflow to be 15.4 × 10 6 m 3 of water, about 1 m water equivalent over the catchment and 2.3 times total annual streamflow. Nitrate is relatively high at higher flow rates in winter, but is low at times of low flow with old water. This reflects both lower nitrate loading in the catchment several decades ago as compared to current intensive dairy farming, and denitrification processes occurring in the older groundwater. Silica, leached from the aquifer material and accumulating in the water in proportion to contact time, is high at times of low streamflow with old water. There was a good correlation between silica concentration and streamwater age, which potentially allows silica concentrations to be used as a proxy for age when calibrated by tritium measurements. This study shows that tritium dating of stream water is possible with single tritium measurements now that bomb-test tritium has effectively disappeared from hydrological systems in New Zealand, without the need for time-series data.

The bedrock controls on catchment mixing, storage, and release have been actively studied in recent years. However, it has been difficult to find neighbouring catchments with sufficiently different and clean expressions of geology to do comparative analysis. Here, we present new data for 16 nested catchments (0.45 to 410 km 2 ) in the Alzette River basin (Luxembourg) that span a range of clean and mixed expressions of schists, phyllites, sandstones, and quartzites to quantify the relationships between bedrock permeability and metrics of water storage and release. We examined 9 years' worth of precipitation and discharge data, and 6 years of fortnightly stable isotope data in streamflow, to explore how bedrock permeability controls (a) streamflow regime metrics, (b) catchment storage, and (c) isotope response and catchment mean transit time (MTT). We used annual and winter precipitation-run-off ratios, as well as average summer and winter precipitation-run-off ratios to characterise the streamflow regime in our 16 study catchments. Catchment storage was then used as a metric for catchment comparison. Water mixing potential of 11 catchments was quantified via the standard deviation in streamflow δD (σδD) and the amplitude ratio (A S /A P ) of annual cycles of δ 18 O in streamflow and precipitation. Catchment MTT values were estimated via both stable isotope signature damping and hydraulic turnover calculations. In our 16 nested catchments, the variance in ratios of summer versus winter average run-off was best explained by bedrock permeability. Whereas active storage (defined here as a measure of the observed maximum interannual variability in catchment storage) ranged from 107 to 373 mm, total catchment storage (defined as the maximum catchment storage connected to the stream network) extended up to~1700 mm (±200 mm). Catchment bedrock permeability was strongly correlated with mixing proxies of σδD in streamflow and δ 18 O A S /A P ratios. Catchment MTT values ranged from 0.5 to 2 years, based on stable isotope signature damping, and from 0.5 to 10 years, based on hydraulic turnover. KEYWORDS bedrock permeability, catchment storage, mean transit time, mesoscale, stable isotope response, streamflow regime

The relationship between streamwater mean residence time (MRT) and landscape characteristics is poorly understood. We used tritium ( 3 H) to define our MRT. We tested the hypothesis that baseflow water MRT increases with increasing absolute catchment size at the Maimai catchments. These catchments are simple hydrologic systems relative to many catchments around the world, with uniformly wet climatic conditions, little seasonality, uniform and nearly impermeable bedrock, steep short hillslopes, shallow soils, and well-characterized hillslope and catchment hydrology. As a result, this is a relatively simple system and an ideal location for new MRT-related hypothesis testing. Whilst hydrologists have used 3 H to estimate water age since the 1960s nuclear testing spike, atmospheric 3 H levels have now approached near background levels and are often complicated by contamination from the nuclear industry. We present results for 3 H sampled from our set of nested catchments in nuclear-industry-free New Zealand. Because of high precision analysis, near-natural atmospheric 3 H levels, and well-characterized rainfall 3 H inputs, we were able to estimate the age of young (i.e. less than 3 years old) waters. Our results showed no correlation between MRT and catchment size. However, MRT was correlated to the median sub-catchment size of the sampled watersheds, as shown by landscape analysis of catchment area accumulation patterns. These preliminary findings suggest that landscape organization, rather than total area, is a first-order control on MRT and points the way forward for more detailed analysis of how landscape organization affects catchment runoff characteristics.