A small stream in a predominantly dairying catchment in the Waikato region of New Zealand was monitored for 2 years at three sites. Total nitrogen (TN) concentrations were up to 7.09 g m -3in winter, with the bulk comprising nitrate nitrogen (NO -3 -N). During summer NO -3 -N was near zero and TN mostly comprised organic nitrogen. Maximum concentrations of total phosphorus (TP) and dissolved reactive phosphorus (DRP) were 1.64 and 0.555 g m -3 , respectively, and peaks coincided with spring and autumn applications of phosphorus fertiliser. Ammoniacal nitrogen concentrations exceeded 1 g m -3 on several occasions and mean concentrations at the three sites were 0.165-0.272 g m -3 . Faecal coliform and enterococci bacteria concentrations were 64-26000 and 7-23000 cfuper 100 Received 23 March 1999; accepted 23 June 1999 ml, respectively. Specific yields of TN and NO -3 -N (35.3 and 30.7 kg ha yr -1 , respectively) were much greater than any previously reported for New Zealand pasture catchments, whereas TP and DRP yields (1.16 and 0.54 kg ha yr -1 , respectively) were more in accord with other studies. Greater use of land treatment of liquid wastes will reduce stream inputs of faecal organisms, NH 4 -N and P. M99015

The potential impacts of climate change and socio-economic change on flow and water quality in rivers worldwide is a key area of interest.

A flood risk model was developed for the Czech Republic to calculate the probability of insured losses from flood events. The model was GIS based, making use of a 100 m horizontal resolution DTM and a network of the major rivers in the country. A review of historical flooding was undertaken to define the worst and most widespread flood events. Synthetic flood events were generated based on a study of the spatial variation in magnitude of river flows from selected historical flood events going back to 1935. A total of 30 synthetic events were generated each providing peak flows at 25 gauging stations throughout the country. The flows were converted into flood levels using rating equations based on information provided by the Czech Hydrological and Meteorological Institute. The extent of and depth of flooding was mapped on a cell by cell basis by applying an automated procedure developed using the grid option within the Arc/Info GIS. The flood depths were combined with maps of the postal codes to define an average flood depth per post code. The model was calibrated using maps of the observed flood extents from 1997 and 2002.

Abstract. The problem of soil erosion is particularly evident in New Zealand, given the combination of coarse‐textured soils, steep relief, high rainfall, and intensification of agriculture. A study was undertaken to assess the effects of land use change on soil erosion and sediment transport for the Ngongotaha catchment in New Zealand's North Island, using a GIS based decision support and modelling system. Model simulations considered the effect of increased catchment area under deer farming and forestry on the amount of sediment delivered to the catchment outlet, averaged over a period of six years. The simulations predicted that sediment loss from land under deer farming was considerably greater than from land under other livestock or forestry. Further model simulations testing best management practices demonstrated that sediment yield could be halved if deer farming was restricted to slopes under 20%.

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25The Mekong delta is recognised as one of the world's most vulnerable mega-deltas, being 26 subject to a range of environmental pressures including sea level rise, increasing population, 27and changes in flows and nutrients from its upland catchment. With changing climate and 28 socioeconomics there is a need to assess how the Mekong catchment will be affected in 29terms of the delivery of water and nutrients into the delta system. Here we apply the 30Integrated Catchment model (INCA) to the whole Mekong River Basin to simulate flow and 31water quality, including nitrate, ammonia, total phosphorus and soluble reactive phosphorus. 32The impacts of climate change on all these variables have been assessed across 24 river 33 reaches ranging from the Himalayas down to the delta in Vietnam. We used the UK Met 34Office PRECIS regionally coupled climate model to downscale precipitation and temperature 35to the Mekong catchment. This was accomplished using the Global Circulation Model GFDL-36CM to provide the boundary conditions under two carbon control strategies, namely 37representative concentration pathways (RCP) 4.5 and a RCP 8.5 scenario. The RCP 4.5 38 scenario represents the carbon strategy required to meet the Paris Accord, which aims to 39 limit peak global temperatures to below a 2 o C rise while seeking to pursue options that limit 40 temperature rise to 1.5 o C. The RCP 8.5 scenario is associated with a larger 3-4 o C rise. In 41 addition, we also constructed a range of socio-economic scenarios to investigate the 42 potential impacts of changing population, atmospheric pollution, economic growth and land 43 use change up to the 2050s. Results of INCA simulations indicate increases in mean flows 44 of up to 24%, with flood flows in the monsoon period increasing by up to 27%, but with 45 increasing periods of drought up to 2050. A shift in the timing of the monsoon is also 46 simulated, with a 4 week advance in the onset of monsoon flows on average. Decreases in 47 nitrogen and phosphorus concentrations occur primarily due to flow dilution, but fluxes of 48 these nutrients also increase by 5%, which reflects the changing flow, land use change and 49 population changes. 50

The Ganga-Brahmaputra-Meghna (GBM) River System, the associated Hooghly River and the Mahanadi River System represent the largest river basins in the world serving a population of over 780 million. The rivers are of vital concern to India and Bangladesh as they provide fresh water for people, agriculture, industry, conservation and support the Delta System in the Bay of Bengal. Future changes in both climate and socio-economics have been investigated to assess whether these will alter river flows and water quality. Climate datasets downscaled from three different Global Climate Models have been used to drive a daily process based flow and water quality model. The results suggest that due to climate change the flows will increase in the monsoon period and also be enhanced in the dry season. However, once socio-economic changes are also considered, increased population, irrigation, water use and industrial development reduce water availability in drought conditions, threatening water supplies and posing a threat to river and coastal ecosystems. This study, as part of the DECCMA (Deltas, vulnerability and Climate Change: Migration and Adaptation) project, also addresses water quality issues, particularly nutrients (N and P) and their transport along the rivers and discharge into the Delta System. Climate will alter flows, increasing flood flows and changing pollution dilution factors in the rivers, as well as other key processes controlling water quality. Socio-economic change will affect water quality, as water diversion strategies, increased population and industrial development alter the water balance and enhance fluxes of nutrients from agriculture, urban centers and atmospheric deposition.

There are ongoing discussions about the appropriate level of complexity and sources of uncertainty in rainfall runoff models. Simulations for operational hydrology, flood forecasting or nutrient transport all warrant different levels of complexity in the modelling approach. More complex model structures are appropriate for simulations of land-cover dependent nutrient transport while more parsimonious model structures may be adequate for runoff simulation. The appropriate level of complexity is also dependent on data availability. Here, we use PERSiST; a simple, semi-distributed dynamic rainfall-runoff modelling toolkit to simulate flows in the Upper Ganges and Brahmaputra rivers. We present two sets of simulations driven by single time series of daily precipitation and temperature using simple (A) and complex (B) model structures based on uniform and hydrochemically relevant land covers respectively. Models were compared based on ensembles of Bayesian Information Criterion (BIC) statistics. Equifinality was observed for parameters but not for model structures. Model performance was better for the more complex (B) structural representations than for parsimonious model structures. The results show that structural uncertainty is more important than parameter uncertainty. The ensembles of BIC statistics suggested that neither structural representation was preferable in a statistical sense. Simulations presented here confirm that relatively simple models with limited data requirements can be used to credibly simulate flows and water balance components needed for nutrient flux modelling in large, data-poor basins.