Thesis (Ph. D.)--University of Canterbury, 2009. / Typescript (photocopy). Includes bibliographical references (p. 214-243). Also available via the World Wide Web.

The management of New Zealand’s freshwater resources has come under increasing pressure from different industrial and environmental stakeholders. Land use change and the pressure it can put on water resources has been a significant issue regarding resource management in New Zealand. A significant mechanism driving land use change has been the growth of forestry, dairy farming, and other agricultural industries. Improvements in agricultural and forestry science and irrigation techniques have allowed new, previously less arable areas of New Zealand to be subject to land use change, such as the conversion of tussock grassland to pasture in steep, mountainous regions in the South Island. Studies regarding the effects of land use change in such catchments, especially with focus on flood hydrology, appear to be limited, despite the importance of managing catchment headwaters to minimise flood risk downstream. The TopNet model was used in this research project to evaluate the potential effects of land use change on flood hydrology in mountain catchments. It is a semi-distributed continuous rainfall-runoff model developed by the National Institute of Water and Atmospheric Research (NIWA). It has been widely used in New Zealand, and applications have included modelling water yield and the effect of climate change in catchment networks. However, it was not developed specifically for predicting flood flows. Hence, testing the model for flood peak prediction in mountainous catchments was also performed, and may show that TopNet can be a useful tool in resource management in New Zealand. The Ahuriri and Pelorus River catchments were used in this investigation. Both are steep catchments located in the South Island. The Ahuriri River catchment, in the Waitaki Basin on the eastern side of the Southern Alps, is a semi-arid catchment dominated by tussock grassland. The surrounding catchments are heavily influenced by infrastructure for hydroelectric power (HEP) generation and more recently irrigation for dairy farming. The Pelorus River catchment is located at the northern end of the South Island. It is primarily covered in native forest, but adjacent catchments are subject to agricultural and forestry development. The ability of the TopNet model for each catchment to predict flood flows were tested using a selection of historical flood events. Rainfall input to the model was at a daily timestep from the virtual climate station network (VCSN), and the method of disaggregating the daily estimate into an hourly rainfall series to be used by the model was found to have a significant influence on flood prediction. Where an accurate historical rainfall record was provided from a rainfall gauge station within the catchments, the disaggregation of the daily rainfall estimate based on the station data produced a significantly more accurate flood prediction when compared to predictions made using a stochastic disaggregation of the daily rainfall estimate. The TopNet models were modified to reflect land use change scenarios: the conversion of tussock grassland to pasture and the afforestation of tussock in the Ahuriri River catchment, and the conversion of forested land to pasture and the harvest of plantation forestry in the Pelorus River catchment. Following a past study into modelling the effects of land use change using TopNet, three key model parameters were modified to reflect each land use scenario: saturated hydraulic conductivity KS, canopy storage capacity, and the canopy enhancement factor. Past studies suggested a wide range of suitable values for KS, although also acknowledged that KS depends heavily on the specific catchment characteristics. A sensitivity analysis showed that KS had a significant influence on flood peak prediction in TopNet. It is recommended that further investigation be conducted into suitable values for KS. TopNet appeared to predict the effect of land use change on flood magnitude in mountainous catchments conservatively. Past studies of land use change suggested that the effect on flood flows should be significant, whereas TopNet generally predicted small changes in flood peaks for the scenarios in each catchment. However, this may suggest that the topography, geology, and soil properties of steep catchments are more important to flood hydrology than land cover. Further investigation into the effect of such catchment characteristics is recommended. Nevertheless, TopNet was shown to have the potential to be a useful tool for evaluating and managing the effects of land use change on the flood hydrology of mountainous catchments in New Zealand.

Characterizing streamflow trends is important for water resources management. Streamflow conditions, and trends thereof, are critical drivers of all aspects of stream geomorphology, sediment and nutrient transport, and ecological processes. Using the non-parametric modified Mann-Kendall test, we analyzed streamflow trends from 1996 to 2022 for the Southern Appalachian (SA) region of the U.S. The forested uplands of the SA receive high amounts of rain and act as a "water tower" for the surrounding lowland area, both of which have experienced higher than average population growth and urban development. For the total of 127 USGS gages with continuous streamflow measurements, we also evaluated precipitation and land change rates and patterns within the upstream contributing areas. Statistical methods (i.e., generalized linear models) were then used to assess any linkages between land cover change (LCC) and streamflow trends. Our results show that 42 drainage areas are experiencing increasing trends in their precipitation, and 1 is experiencing a negative trend. A total of 71 drainage areas are experiencing increasing trends in either their annual streamflow minimums, maximums, medians, or variability, with some experiencing changes in multiple. From our models, it is suggested that agricultural expansion is associated with increasing minimum streamflow trends, but increasing precipitation is also positively linked. With this information, water managers would be aware of which areas are experiencing changes in streamflow amounts from LCC or precipitation and could then apply this in planning and predictions. / Master of Science / Water availability is important for resources management. Streamflow is a measure of available surface water and is an important component in the hydrological cycle. Using the non-parametric modified Mann-Kendall test, we analyzed streamflow trends from 1996 to 2022 for the Southern Appalachian (SA) region of the U.S. The forested uplands of the SA receive high amounts of rain and act as a "water tower" for the surrounding lowland area, both of which have experienced higher than average population growth and city expansion. For the total of 127 USGS gages with continuous streamflow measurements, we also evaluated precipitation and land cover change rates within the area upstream of the gage (or drainage/contributing area). Statistical methods (i.e., generalized linear models) were then used to assess any linkages between land cover change (LCC) and streamflow trends. Our results show that 42 drainage areas are experiencing increasing trends in their precipitation, and 1 is experiencing a negative trend. A total of 71 drainage areas are experiencing increasing trends in either their annual streamflow minimums, maximums, medians, or variability, with some experiencing changes in multiple. From our models, it is suggested that agricultural expansion is associated with increasing minimum streamflow trends, but increasing precipitation is also positively linked. With this information, water managers would be aware of which areas are experiencing changes in streamflow amounts from LCC or precipitation and could then apply this in planning and predictions.

abstract: This doctoral thesis investigates the predictability characteristics of floods and flash floods by coupling high resolution precipitation products to a distributed hydrologic model. The research hypotheses are tested at multiple watersheds in the Colorado Front Range (CFR) undergoing warm-season precipitation. Rainfall error structures are expected to propagate into hydrologic simulations with added uncertainties by model parameters and initial conditions. Specifically, the following science questions are addressed: (1) What is the utility of Quantitative Precipitation Estimates (QPE) for high resolution hydrologic forecasts in mountain watersheds of the CFR?, (2) How does the rainfall-reflectivity relation determine the magnitude of errors when radar observations are used for flood forecasts?, and (3) What are the spatiotemporal limits of flood forecasting in mountain basins when radar nowcasts are used into a distributed hydrological model?. The methodology consists of QPE evaluations at the site (i.e., rain gauge location), basin-average and regional scales, and Quantitative Precipitation Forecasts (QPF) assessment through regional grid-to-grid verification techniques and ensemble basin-averaged time series. The corresponding hydrologic responses that include outlet discharges, distributed runoff maps, and streamflow time series at internal channel locations, are used in light of observed and/or reference data to diagnose the suitability of fusing precipitation forecasts into a distributed model operating at multiple catchments. Results reveal that radar and multisensor QPEs lead to an improved hydrologic performance compared to simulations driven with rain gauge data only. In addition, hydrologic performances attained by satellite products preserve the fundamental properties of basin responses, including a simple scaling relation between the relative spatial variability of runoff and its magnitude. Overall, the spatial variations contained in gridded QPEs add value for warm-season flood forecasting in mountain basins, with sparse data even if those products contain some biases. These results are encouraging and open new avenues for forecasting in regions with limited access and sparse observations. Regional comparisons of different reflectivity -rainfall (Z-R) relations during three summer seasons, illustrated significant rainfall variability across the region. Consistently, hydrologic errors introduced by the distinct Z-R relations, are significant and proportional (in the log-log space) to errors in precipitation estimations and stream flow magnitude. The use of operational Z-R relations without prior calibration may lead to wrong estimation of precipitation, runoff magnitude and increased flood forecasting errors. This suggests that site-specific Z-R relations, prior to forecasting procedures, are desirable in complex terrain regions. Nowcasting experiments show the limits of flood forecasting and its dependence functions of lead time and basin scale. Across the majority of the basins, flood forecasting skill decays with lead time, but the functional relation depends on the interactions between watershed properties and rainfall characteristics. Both precipitation and flood forecasting skills are noticeably reduced for lead times greater than 30 minutes. Scale dependence of hydrologic forecasting errors demonstrates reduced predictability at intermediate-size basins, the typical scale of convective storm systems. Overall, the fusion of high resolution radar nowcasts and the convenient parallel capabilities of the distributed hydrologic model provide an efficient framework for generating accurate real-time flood forecasts suitable for operational environments. / Dissertation/Thesis / Ph.D. Civil and Environmental Engineering 2012

No description available.

L'économie du Pakistan, fondée sur l'agriculture, est hautement dépendante de l'approvisionnement en eau issu de la fonte de la neige et des glaciers du Haut Bassin de l'Indus (UIB) qui s'étend sur les chaînes de l'Himalaya, du Karakoram et de l'Hindukush. Il est par conséquent essentiel pour la gestion des ressources en eau d'appréhender la dynamique de la cryosphère (neige et glace), ainsi que les régimes hydrologiques de cette région dans le contexte de scénarios de changement climatique. La base de données satellitaire du produit de couverture neigeuse MODIS MOD10A2 a été utilisée de mars 2000 à décembre 2009 pour analyser la dynamique du couvert neigeux de l'UIB. Les données journalières de débits à 13 stations hydrométriques et de précipitation et température à 18 postes météorologiques ont été exploitées sur des périodes variables selon les stations pour étudier le régime hydro-climatique de la région. Les analyses satellitaires de la couverture neigeuse et glaciaire suggèrent une très légère extension de la cryosphère au cours de la dernière décade (2000‒2009) en contradiction avec la rapide fonte des glaciers observée dans la plupart des régions du monde. Le modèle « Snowmelt Runoff » (SRM), associé aux produits neige du capteur MODIS a été utilisé avec succès pour simuler les débits journaliers et étudier les impacts du changement climatique sur ces débits dans les sous-bassins à contribution nivo-glaciaire de l'UIB. L'application de SRM pour différents scénarios futurs de changement climatique indique un doublement des débits pour le milieu du siècle actuel. La variation des écoulement de l'UIB, la capacité décroissante des réservoirs existants (barrage de Tarbela) à cause de la sédimentation, ainsi que la demande croissante pour les différents usages de l'eau, laissent penser que de nouveaux réservoirs sont à envisager pour stocker les écoulements d'été et répondre aux nécessités de l'irrigation, de la production hydro-électrique, de la prévention des crues et de l'alimentation en eau domestique. / Agriculture based economy of Pakistan is highly dependent on the snow and glacier melt water supplies from the Upper Indus River Basin (UIB), situated in the Himalaya, Karakoram and Hindukush ranges. It is therefore essential to understand the cryosphere (snow and ice) dynamics and hydrological regime of this area under changed climate scenarios, for water resource management. The MODIS MOD10A2 remote-sensing database of snow cover products from March 2000 to December 2009 was selected to analyse the snow cover dynamics in the UIB. A database of daily flows from 13 hydrometric stations and climate data (precipitation and temperature) from 18 gauging stations, over different time periods for different stations, was made available to investigate the hydro-climatological regime in the area. Analysis of remotely sensed cryosphere (snow and ice cover) data during the last decade (2000‒2009) suggest a rather slight expansion of cryosphere in the area in contrast to most of the regions in the world where glaciers are melting rapidly. The Snowmelt Runoff Model (SRM) integrated with MODIS remote-sensing snow cover products was successfully used to simulate the daily discharges and to study the climate change impact on these discharges in the snow and glacier fed sub-catchments of UIB. The application of the SRM under future climate change scenarios indicates a doubling of summer runoff until the middle of this century. This variation in the Upper Indus River flow, decreasing capacity of existing reservoirs (Tarbela Dam) by sedimentation and the increasing demand of water uses suggests that new reservoirs shall be planned for summer flow storage to meet with the needs of irrigation supply, increasing power generation demand, flood control and water supply.

In arid regions, mountain catchments are the major contributor to the total natural water yield. Due to generally low groundwater tables, subsurface underflow - referred to as mountain-front recharge - is important in distinction to the surface runoff at the mountain front. The extent of the groundwater basin is hereby often vague. Approaches to assess mountain-front recharge are mostly based on groundwater data and integrate over time and space. This, however, cannot provide prognostic and time-dependent estimates of subsurface inflow to the adjacent alluvial basin aquifer. Consequently, the proposed strategy builds on rainfall based approaches. Temporal and spatial resolution is in this case mostly limited by data scarcity regarding hydrological characteristics of the catchment area and high-resolution rainfall data. The proposed novel strategy combines three approaches to tackle these challenges. A newly developed conceptual hydrologic model provides time-dependent estimates based on fully distributed monthly rainfall. For distinct response units and seasons, non-linear relationships between rainfall and recharge describe the hydrogeologic response. The derivation of the response functions is based on a mass balance and considers the principal recharge mechanisms. Parameterisation makes use of available expert knowledge on geomorphology and seasonal rainfall characteristics. As an efficient tool to assess uncertainties, fuzzy arithmetic is used for complementary long-term average water balance estimates. This technique allows considering fuzziness in rainfall input, crop water use in mountain oases, and best available assumptions on recharge as portion of rainfall. Uncertainty regarding the potential, albeit unknown extent of groundwater basins is portrayed based on continuous surfaces which represent the degree of membership to a distinct geographical entity (termed as fuzzy regions). Distinct subsets of these fuzzy regions represent potential groundwater basins for water balance assessment. The proposed strategy was applied on the large scale in an arid karst mountain range in northern Oman. The two complementary assessment approaches result in similar ranges of values. They are in good agreement with inversely computed inflow to a steady state groundwater model for the adjacent basin aquifer. The results of the conceptual hydrologic model are confirmed by the plausibility of average recharge rates for distinct response units and seasons. This shows that less intense winter rainfall contributes mainly to groundwater recharge. Uncertainties due to the vague extent of the groundwater basin are about 30 % of the total mean annual value. An option to mitigate this uncertainty is the complementary consideration of adjacent aquifer systems in future studies. Hydrogeologic survey and observation of groundwater levels in the alluvial basin aquifer in near distance to the mountains is a way to underpin these findings in future studies. This recommenddation applies not only to the discussed study area, but also to mountain block systems in general. / In ariden Gebieten haben Gebirgseinzugsgebiete einen wesentlichen Anteil am gesamten natürlichen Wasserdargebot. Aufgrund i. Allg. tief liegender Grundwasserspiegel ist - in Abgrenzung zum Oberflächenabfluss am Gebirgsrand - auch der unterirdische Abstrom (mountain-front recharge) von besonderer Bedeutung. Die Ausdehnung des unterirdischen Einzugsgebiets ist dabei oft vage. Ansätze zur Abschätzung des mountain-front recharge basieren meist auf Grundwasserdaten und integrieren in Zeit und Raum. Damit können allerdings keine prognostischen oder zeitabhängigen Schätzungen für den Zustrom zur benachbarten alluvialen Aquifer gemacht werden. Daher wird im folgenden ein niederschlagsbasierter Ansatz vorgeschlagen. Das vorgeschlagene neue Konzept kombiniert drei Ansätze, um den genannten Herausforderungen zu begegnen. Mit einem neu entwickelten konzeptionellen hydrologischen Modell auf Basis verteilter Niederschläge werden monatliche Werte für die Grundwasserneubildung bereitgestellt. Es basiert auf nicht-linearen Beziehungen zwischen Niederschlag und Grundwasserneubildung für definierte hydrologisch homogene Einheiten und Jahreszeiten. Deren Ableitung basiert auf einer Massenbilanz und berücksichtigt die wesentlichen Neubildungsmechanismen. Die Parametrisierung basiert auf Expertenwissen zu Geomorphologie und Niederschlagscharakteristika. Fuzzy Arithmetik wird zur Berücksichtigung von Unsicherheiten in einer ergänzenden mittleren jährlichen Wasserbilanz verwendet. Damit können Unschärfen im Niederschlagsinput, beim Pflanzenwasserbedarf in Gebirgsoasen und best verfügbaren Schätzungen der Neubildung als Bruchteil des Niederschlags effizient berücksichtigt werden. Mittels kontinuierlicher Oberflächen, die den Grad der Zugehörigkeit zu einer bestimmten geographischen Entität anzeigen (fuzzy regions) werden Unsicherheiten in der räumlichen Ausdehnung der unterirdischen Einzugsgebiete beschrieben. Definierte Teilmengen dieser fuzzy regions werden dann bei den Wasserhaushaltsbetrachtungen als potentielle Grundwassereinzugsgebiete verwendet. Der vorgeschlagene Ansatz wurde in einer ariden, teils verkarsteten Gebirgsregion im Norden des Sultanats Oman angewendet. Die beiden sich ergänzenden Ansätze zur Abschätzung der Grundwasserneubildung ergaben im langjährigen Mittel vergleichbare Werte. Diese stimmten auch gut mit den Ergebnissen einer inversen Grundwassermodellierung überein. Die Plausibilität der Neubildungsraten für bestimmte hydrologisch homogene Einheiten und Jahreszeiten spricht für die Verlässlichkeit der Ergebnisse des konzeptionellen hydrologischen Modells. Offensichtlich tragen insbesondere die weniger intensiven Winterniederschläge wesentlich zur Grundwasserneubildung bei. Die Unsicherheiten bezüglich der Ausdehnung des Grundwassereinzugsgebiets belaufen sich auf ca. 30 % des mittleren jährlichen Dargebots. Die komplementäre Betrachtung benachbarter Grundwassereinzugsgebiete ist ein denkbarer Weg, diese Unsicherheit in Zukunft zu reduzieren. Ein wesentlicher Beitrag um die Ergebnisse dieser Studie zukünftig weiter zu untermauern wären hydrogeologische Erkundung und Beobachtung von Grundwasserständen im alluvialen Aquifer, insbesondere nahe dem Gebirgsrand. Diese Empfehlung gilt über dieses Fallbeispiel hinaus für vergleichbare Systeme, in denen ein Gebirgseinzugsgebiet den Aquifer in der angrenzende Ebene speist.

In arid regions, mountain catchments are the major contributor to the total natural water yield. Due to generally low groundwater tables, subsurface underflow - referred to as mountain-front recharge - is important in distinction to the surface runoff at the mountain front. The extent of the groundwater basin is hereby often vague. Approaches to assess mountain-front recharge are mostly based on groundwater data and integrate over time and space. This, however, cannot provide prognostic and time-dependent estimates of subsurface inflow to the adjacent alluvial basin aquifer. Consequently, the proposed strategy builds on rainfall based approaches. Temporal and spatial resolution is in this case mostly limited by data scarcity regarding hydrological characteristics of the catchment area and high-resolution rainfall data. The proposed novel strategy combines three approaches to tackle these challenges. A newly developed conceptual hydrologic model provides time-dependent estimates based on fully distributed monthly rainfall. For distinct response units and seasons, non-linear relationships between rainfall and recharge describe the hydrogeologic response. The derivation of the response functions is based on a mass balance and considers the principal recharge mechanisms. Parameterisation makes use of available expert knowledge on geomorphology and seasonal rainfall characteristics. As an efficient tool to assess uncertainties, fuzzy arithmetic is used for complementary long-term average water balance estimates. This technique allows considering fuzziness in rainfall input, crop water use in mountain oases, and best available assumptions on recharge as portion of rainfall. Uncertainty regarding the potential, albeit unknown extent of groundwater basins is portrayed based on continuous surfaces which represent the degree of membership to a distinct geographical entity (termed as fuzzy regions). Distinct subsets of these fuzzy regions represent potential groundwater basins for water balance assessment. The proposed strategy was applied on the large scale in an arid karst mountain range in northern Oman. The two complementary assessment approaches result in similar ranges of values. They are in good agreement with inversely computed inflow to a steady state groundwater model for the adjacent basin aquifer. The results of the conceptual hydrologic model are confirmed by the plausibility of average recharge rates for distinct response units and seasons. This shows that less intense winter rainfall contributes mainly to groundwater recharge. Uncertainties due to the vague extent of the groundwater basin are about 30 % of the total mean annual value. An option to mitigate this uncertainty is the complementary consideration of adjacent aquifer systems in future studies. Hydrogeologic survey and observation of groundwater levels in the alluvial basin aquifer in near distance to the mountains is a way to underpin these findings in future studies. This recommenddation applies not only to the discussed study area, but also to mountain block systems in general.:1 Mountains - 'water towers' for water resources systems in arid regions 2 Mountain hydrology and water resources assessment 2.1 Mountain hydrology and mountain-front recharge 2.2 Essential aspects to advance mountain hydrology 2.2.1 Rainfall characteristics and options for data acquisition 2.2.2 Groundwater-surface water interactions and availability of reference values 2.3 Approaches to the assessment of mountain-front recharge 2.3.1 Options to assess groundwater recharge 2.3.2 Arid zone water balance modelling - options and limitations 2.3.3 Key components for assessing mountain-front recharge 2.4 Linear reservoir models to describe base flow recession 3 Approaches to deal with uncertainty with a special focus on fuzzy sets 3.1 Probability based uncertainty assessment versus fuzzy reasoning 3.2 Fuzzy sets and related methods 3.3 Ranges of application in hydrology and water resources management 4 A novel strategy for estimating groundwater recharge in arid mountain regions 5 Fuzzy-based tools to portray uncertainties in water balance assessment 5.1 Fuzzy Recharge Areas: From qualitative data to quantitative conclusions 5.1.1 The concept of the Fuzzy Recharge Areas 5.1.2 Consideration of adjacent basins 5.2 Water balance assessment based on fuzzy arithmetic 5.2.1 Outline of the calculation procedure 5.2.2 Implementation of the fuzzy arithmetic operators 5.2.3 Implementation of the regionalisation approach APLIS 6 A conceptual hydrologic model to assess mountain-front recharge 6.1 Basic idea 6.2 Model structure 6.3 Calculation procedure using histograms of rainfall depths 6.4 Non-linear seasonal rainfall-recharge relationships 6.4.1 Derivation of the rainfall-recharge relationships 6.4.2 Sensitivity analysis 6.4.3 Response functions based on extreme parameter sets 6.5 Subsurface routing based on linear reservoir models 7 Case Study: Groundwater recharge assessment for the Barka Region (Oman) 7.1 Study area 7.1.1 Topography 7.1.2 Climate 7.1.3 Evapotranspiration 7.1.4 Rainfall characteristics 7.1.5 Geology and Hydrogeology 7.1.6 Soils 7.1.7 Runoff characteristics 7.1.8 Vegetation and irrigated agriculture in mountain oases 7.2 Recharge mechanisms in the study area 7.3 Assessment of mountain-front recharge - Methodology 7.3.1 Data Processing of Fuzzy Recharge Areas 7.3.2 Determining response units 7.3.3 Water use in mountain oases 7.3.4 Long-term average considerations based on fuzzy arithmetic 7.3.5 Time-dependent assessment using the conceptual hydrologic model 7.4 Assessment of mountain-front recharge - Results 7.4.1 Long-term average considerations 7.4.2 Time-dependent estimates 7.5 Consideration of uncertainties 7.6 Discussion & Conclusions 7.6.1 Water resources assessment in the study area 7.6.2 Modelling approaches 8 Summary 9 Prospects for future work List of Figures List of Tables List of Symbols List of Abbreviations Appendix A: Sensitivity of the response function to variations of the different model parameters B: Histograms of subsurface outflow at the mountain front QMFR based on different parameterisations of the conceptual hydrologic model References / In ariden Gebieten haben Gebirgseinzugsgebiete einen wesentlichen Anteil am gesamten natürlichen Wasserdargebot. Aufgrund i. Allg. tief liegender Grundwasserspiegel ist - in Abgrenzung zum Oberflächenabfluss am Gebirgsrand - auch der unterirdische Abstrom (mountain-front recharge) von besonderer Bedeutung. Die Ausdehnung des unterirdischen Einzugsgebiets ist dabei oft vage. Ansätze zur Abschätzung des mountain-front recharge basieren meist auf Grundwasserdaten und integrieren in Zeit und Raum. Damit können allerdings keine prognostischen oder zeitabhängigen Schätzungen für den Zustrom zur benachbarten alluvialen Aquifer gemacht werden. Daher wird im folgenden ein niederschlagsbasierter Ansatz vorgeschlagen. Das vorgeschlagene neue Konzept kombiniert drei Ansätze, um den genannten Herausforderungen zu begegnen. Mit einem neu entwickelten konzeptionellen hydrologischen Modell auf Basis verteilter Niederschläge werden monatliche Werte für die Grundwasserneubildung bereitgestellt. Es basiert auf nicht-linearen Beziehungen zwischen Niederschlag und Grundwasserneubildung für definierte hydrologisch homogene Einheiten und Jahreszeiten. Deren Ableitung basiert auf einer Massenbilanz und berücksichtigt die wesentlichen Neubildungsmechanismen. Die Parametrisierung basiert auf Expertenwissen zu Geomorphologie und Niederschlagscharakteristika. Fuzzy Arithmetik wird zur Berücksichtigung von Unsicherheiten in einer ergänzenden mittleren jährlichen Wasserbilanz verwendet. Damit können Unschärfen im Niederschlagsinput, beim Pflanzenwasserbedarf in Gebirgsoasen und best verfügbaren Schätzungen der Neubildung als Bruchteil des Niederschlags effizient berücksichtigt werden. Mittels kontinuierlicher Oberflächen, die den Grad der Zugehörigkeit zu einer bestimmten geographischen Entität anzeigen (fuzzy regions) werden Unsicherheiten in der räumlichen Ausdehnung der unterirdischen Einzugsgebiete beschrieben. Definierte Teilmengen dieser fuzzy regions werden dann bei den Wasserhaushaltsbetrachtungen als potentielle Grundwassereinzugsgebiete verwendet. Der vorgeschlagene Ansatz wurde in einer ariden, teils verkarsteten Gebirgsregion im Norden des Sultanats Oman angewendet. Die beiden sich ergänzenden Ansätze zur Abschätzung der Grundwasserneubildung ergaben im langjährigen Mittel vergleichbare Werte. Diese stimmten auch gut mit den Ergebnissen einer inversen Grundwassermodellierung überein. Die Plausibilität der Neubildungsraten für bestimmte hydrologisch homogene Einheiten und Jahreszeiten spricht für die Verlässlichkeit der Ergebnisse des konzeptionellen hydrologischen Modells. Offensichtlich tragen insbesondere die weniger intensiven Winterniederschläge wesentlich zur Grundwasserneubildung bei. Die Unsicherheiten bezüglich der Ausdehnung des Grundwassereinzugsgebiets belaufen sich auf ca. 30 % des mittleren jährlichen Dargebots. Die komplementäre Betrachtung benachbarter Grundwassereinzugsgebiete ist ein denkbarer Weg, diese Unsicherheit in Zukunft zu reduzieren. Ein wesentlicher Beitrag um die Ergebnisse dieser Studie zukünftig weiter zu untermauern wären hydrogeologische Erkundung und Beobachtung von Grundwasserständen im alluvialen Aquifer, insbesondere nahe dem Gebirgsrand. Diese Empfehlung gilt über dieses Fallbeispiel hinaus für vergleichbare Systeme, in denen ein Gebirgseinzugsgebiet den Aquifer in der angrenzende Ebene speist.:1 Mountains - 'water towers' for water resources systems in arid regions 2 Mountain hydrology and water resources assessment 2.1 Mountain hydrology and mountain-front recharge 2.2 Essential aspects to advance mountain hydrology 2.2.1 Rainfall characteristics and options for data acquisition 2.2.2 Groundwater-surface water interactions and availability of reference values 2.3 Approaches to the assessment of mountain-front recharge 2.3.1 Options to assess groundwater recharge 2.3.2 Arid zone water balance modelling - options and limitations 2.3.3 Key components for assessing mountain-front recharge 2.4 Linear reservoir models to describe base flow recession 3 Approaches to deal with uncertainty with a special focus on fuzzy sets 3.1 Probability based uncertainty assessment versus fuzzy reasoning 3.2 Fuzzy sets and related methods 3.3 Ranges of application in hydrology and water resources management 4 A novel strategy for estimating groundwater recharge in arid mountain regions 5 Fuzzy-based tools to portray uncertainties in water balance assessment 5.1 Fuzzy Recharge Areas: From qualitative data to quantitative conclusions 5.1.1 The concept of the Fuzzy Recharge Areas 5.1.2 Consideration of adjacent basins 5.2 Water balance assessment based on fuzzy arithmetic 5.2.1 Outline of the calculation procedure 5.2.2 Implementation of the fuzzy arithmetic operators 5.2.3 Implementation of the regionalisation approach APLIS 6 A conceptual hydrologic model to assess mountain-front recharge 6.1 Basic idea 6.2 Model structure 6.3 Calculation procedure using histograms of rainfall depths 6.4 Non-linear seasonal rainfall-recharge relationships 6.4.1 Derivation of the rainfall-recharge relationships 6.4.2 Sensitivity analysis 6.4.3 Response functions based on extreme parameter sets 6.5 Subsurface routing based on linear reservoir models 7 Case Study: Groundwater recharge assessment for the Barka Region (Oman) 7.1 Study area 7.1.1 Topography 7.1.2 Climate 7.1.3 Evapotranspiration 7.1.4 Rainfall characteristics 7.1.5 Geology and Hydrogeology 7.1.6 Soils 7.1.7 Runoff characteristics 7.1.8 Vegetation and irrigated agriculture in mountain oases 7.2 Recharge mechanisms in the study area 7.3 Assessment of mountain-front recharge - Methodology 7.3.1 Data Processing of Fuzzy Recharge Areas 7.3.2 Determining response units 7.3.3 Water use in mountain oases 7.3.4 Long-term average considerations based on fuzzy arithmetic 7.3.5 Time-dependent assessment using the conceptual hydrologic model 7.4 Assessment of mountain-front recharge - Results 7.4.1 Long-term average considerations 7.4.2 Time-dependent estimates 7.5 Consideration of uncertainties 7.6 Discussion & Conclusions 7.6.1 Water resources assessment in the study area 7.6.2 Modelling approaches 8 Summary 9 Prospects for future work List of Figures List of Tables List of Symbols List of Abbreviations Appendix A: Sensitivity of the response function to variations of the different model parameters B: Histograms of subsurface outflow at the mountain front QMFR based on different parameterisations of the conceptual hydrologic model References