Precipitation-runoff Relationship Research Articles

Effects of multi-year droughts on the precipitation-runoff relationship: An integrated analysis of meteorological, hydrological, and compound droughts

Multi-year droughts cause severe water scarcity, ecosystem collapse, and significant socio-economic losses. Understanding the impact of these droughts on the precipitation-runoff (PR) relationship is crucial for managing water resources and mitigating drought-related damage. However, previous research focuses mainly on the influence of multi-year meteorological droughts on the PR relationship, neglecting multi-year hydrological droughts and their compound effects. Studies also generally overlook the speed and volume of precipitation-to-runoff conversion, considering only the quality of PR relationship. Furthermore, there is a lack of effective simulation methods using the intensity of multi-year droughts to understand the PR relationship. To address these gaps, this study defines multi-year compound droughts and employs three key parameters, namely, the determination of coefficient (R2), linear regression slope, and runoff coefficient, to assess the impact of multi-year droughts on changes in the PR relationship's quality, precipitation-to-runoff conversion speed, and runoff volume. A random forest model is then employed to simulate these parameters based on the intensity of multi-year droughts. Case studies conducted in three basins within the arid-semiarid region of northern China as well as in three basins in the humid region of southern China reveal that multi-year droughts result in water shortages and have a significant impact on the PR relationship. Among different types of multi-year droughts, hydrological droughts have the greatest impact on the PR relationship, followed by compound and meteorological droughts, respectively. During meteorological, hydrological, and compound drought periods, the average rainfall (runoff) is lower by 26 – 30% (33 – 58%), 18 – 28% (36 – 60%), and 18 – 26% (30 – 58%) respectively compared to non-drought periods. Furthermore, the PR relationship remains stable in the rivers of the humid region in southern China. In contrast, the Loess Plateau region in northern China exhibits a substantial decrease in both the speed and volume of precipitation-to-runoff conversion. Interestingly, this decline is not accompanied by a significant increase in the R2 of the PR relationship. The random forest approach effectively simulates key parameters of the PR relationship based on the intensity of multi-year droughts. These findings hold crucial implications for the long-term regional water resource planning.

Insights into runoff changes in the source region of Yellow River under frozen ground degradation

The source region of the Yellow River (SRYR) in the northeastern Tibetan Plateau is critical for supplying water resources to downstream areas. However, streamflow in the SRYR declined despite a slight increase in precipitation during the past few decades. The SRYR experienced significant frozen ground degradation with climate warming, but how frozen ground degradation influences runoff remains unclear. This study investigated the changes of the precipitation-runoff relationship using the double-mass curve method and examined the impact of long-term spatiotemporal changes in frozen ground on the water balance components using the geomorphology-based eco-hydrological model (GBEHM). The results showed that the precipitation-runoff relationship changed significantly since 1989 in the SRYR from 1960 to 2019. In the same period, the areal mean value of the maximum thickness of seasonally frozen ground (MTSFG) decreased by 0.10 m/10a and the areal mean active layer thickness (ALT) of permafrost increased by 0.06 m/10a. Besides, 21.0 % of the entire SRYR has degraded from permafrost to seasonally frozen ground (SFG). Runoff decreased mainly in the region with elevation below 4200 m, where the evapotranspiration increase exceeded the precipitation increase. Frozen ground degradation significantly altered the hydrological processes, which is reflected by the increased subsurface runoff and the decreased surface runoff. The total water storage increased by 2.9 mm/a in the permafrost region due to the increase in active layer thickness and by 5.7 mm/a in the degradation region where permafrost completely thawed during 1960–2019. The runoff seasonality was also altered, being indicated as an increase in winter runoff. These findings help provide a better understanding of the runoff change under climate warming in permafrost-affected regions and provide insights into future water resources management in the Yellow River basin under the climate warming.

Quantifying multi-year hydrological memory with Catchment Forgetting Curves

Abstract. A climatic anomaly can potentially affect the hydrological behaviour of a catchment for several years. This article presents a new approach to quantifying this multi-year hydrological memory, using exclusively streamflow and climate data. Rather than providing a single value of catchment memory, we aim to describe how this memory fades over time. The precipitation–runoff relationship is analyzed through the concept of elasticity. Elasticity quantifies the change in one quantity caused by the change in another quantity. We analyze the elasticity of the relation between the annual anomalies of runoff yield and humidity index. We identify Catchment Forgetting Curves (CFC) to quantify multi-year catchment memory, considering not only the current year's humidity anomaly but also the anomalies of the preceding years. The variability of CFCs is investigated on a set of 158 Swedish and 527 French catchments. As expected, French catchments overlying large aquifers exhibit a long memory, i.e., with the impact of climate anomalies detected over several years. In Sweden, the expected effect of the lakes is less clear. For both countries, a relatively strong relationship between the humidity index and memory is identified, with drier regions exhibiting longer memory. Taking into account the multi-year memory has significantly improved the elasticity analysis for 15 % of the catchments. Our work thus underlines the need to account for catchment memory in order to produce meaningful and geographically coherent elasticity indices.

Evaporation enhancement drives the European water-budget deficit during multi-year droughts

Abstract. In a warming climate, periods with lower than average precipitation will increase in frequency and intensity. During such periods, known as meteorological droughts, the decline in annual runoff may be proportionally larger than the corresponding decline in precipitation. Reasons behind this exacerbation of runoff deficit during dry periods remain largely unknown, and this challenges the predictability of when this exacerbation will occur in the future and how intense it will be. In this work, we tested the hypothesis that runoff deficit exacerbation during droughts is a common feature across climates, driven by evaporation enhancement. We relied on multidecadal records of streamflow and precipitation for more than 200 catchment areas across various European climates, which distinctively show the emergence of similar periods of exacerbated runoff deficit identified in previous studies, i.e. runoff deficit on the order of −20 % to −40 % less than what expected from precipitation deficits. The magnitude of this exacerbation is two to three times larger for basins located in dry regions than for basins in wet regions, and is qualitatively correlated with an increase in annual evaporation during droughts, in the order of +11 % and +33 % over basins characterized by energy-limited and water-limited evaporation regimes, respectively. Thus, enhanced atmospheric and vegetation demand for moisture during dry periods induces a nonlinear precipitation-runoff relationship for low-flow regimes, which results in an unexpectedly large decrease in runoff during periods of already low water availability. Forecasting onset, magnitude, and duration of these drops in runoff have paramount societal and ecological implications, especially in a warming climate, given their supporting role for safeguarding water, food, and energy. The outcome that water basins are prone to this exacerbation of runoff deficit for various climates and evaporation regimes makes further understanding of its patterns of predictability an urgent priority for water-resource planning and management in a warming and drier climate.

Drivers of drought-induced shifts in the water balance through a Budyko approach

Abstract. An inconsistent relationship between precipitation and runoff has been observed between drought and non-drought periods, with less runoff usually observed during droughts than would be expected based solely on precipitation deficit. Predictability of these shifts in the precipitation–runoff relationship is still challenging, largely because the underlying hydrologic mechanisms are poorly constrained. Using 30 years of data for 14 basins in California, we show how the Budyko framework can be leveraged to decompose shifts in precipitation versus runoff during droughts into “regime” shifts, which result from changes in the aridity index along the same Budyko curve, and “partitioning shifts”, which imply a change in the Budyko parameter ω and thus in the relationship among water balance components that governs partitioning of available water. Regime shifts are primarily due to measurable interannual changes in precipitation or temperature, making them predictable based on drought conditions. Partitioning shifts involve further nonlinear and indirect catchment feedbacks to drought conditions and are thus harder to predict a priori. We show that regime shifts dominate changes in absolute runoff during droughts but that gains or losses due to partitioning shifts are still significant. Low aridity, high baseflow, a shift from snow to rain, and resilience of high-elevation runoff correlate with higher annual runoff during droughts than would be predicted by the precipitation–runoff ratio during non-drought years. Differentiating between these shifts in the precipitation–runoff relationship using a Budyko approach will help water resource managers, particularly in arid, drought-prone regions, to better project runoff magnitudes during droughts based on available climate data and, furthermore, understand under what circumstances and to what extent their forecasts may be less reliable due to nonlinear basin–climate feedbacks.

Areal Precipitation Coverage Ratio for Enhanced AI Modelling of Monthly Runoff: A New Satellite Data-Driven Scheme for Semi-Arid Mountainous Climate

Satellite remote sensing provides useful gridded data for the conceptual modelling of hydrological processes such as precipitation–runoff relationship. Structurally flexible and computationally advanced AI-assisted data-driven (DD) models foster these applications. However, without linking concepts between variables from many grids, the DD models can be too large to be calibrated efficiently. Therefore, effectively formulized, collective input variables and robust verification of the calibrated models are desired to leverage satellite data for the strategic DD modelling of catchment runoff. This study formulates new satellite-based input variables, namely, catchment- and event-specific areal precipitation coverage ratios (CCOVs and ECOVs, respectively) from the Global Precipitation Mission (GPM) and evaluates their usefulness for monthly runoff modelling from five mountainous Karkheh sub-catchments of 5000–43,000 km2 size in west Iran. Accordingly, 12 different input combinations from GPM and MODIS products were introduced to a generalized deep learning scheme using artificial neural networks (ANNs). Using an adjusted five-fold cross-validation process, 420 different ANN configurations per fold choice and 10 different random initial parameterizations per configuration were tested. Runoff estimates from five hybrid models, each an average of six top-ranked ANNs based on six statistical criteria in calibration, indicated obvious improvements for all sub-catchments using the new variables. Particularly, ECOVs were most efficient for the most challenging sub-catchment, Kashkan, having the highest spacetime precipitation variability. However, better performance criteria were found for sub-catchments with lower precipitation variability. The modelling performance for Kashkan indicated a higher dependency on data partitioning, suggesting that long-term data representativity is important for modelling reliability.

Hydrological basis of the Devils Lake, North Dakota (USA), terminal lake flood disaster

Devils Lake, a terminal lake in northeast North Dakota (USA), has experienced catastrophic flooding since 1993. From January 31, 1993, to December 31, 2014, lake level rose from 433.62 to 442.44 m, lake area expanded from 179.9 to 653.5 km2, and lake volume increased from 0.70 to 3.80 km3. More than $1 billion ($USD) has been spent in government payments to mitigate direct, primary, tangible flood damages. This paper provides a case study of the hydrological basis of the Devils Lake flood disaster. The unique geomorphic setting, paleoclimatic record, and hydroclimatic conditions of the region are summarized, and a wide range of hydroclimatic data is examined to provide a broad understanding of the physical basis of the flood disaster. The primary cause of the disaster was a transition to a sustained wetter climate that resulted in a dramatic response in basin hydrological variables in 1993. The transition from a long-term dry period to a long-term wet period caused the lake water budget to begin to change from an atmosphere-controlled water budget dominated by precipitation input to an amplifier lake water budget dominated by surface runoff input to the lake. Other important hydrological factors include a nonlinear precipitation–runoff relationship following the long-term drought, fill-spill and fill-merge hydrological behavior that is characteristic of wetland complexes, an increase in the lake area-to-basin area ratio, and the critical role of frozen soils in controlling infiltration and runoff production of spring snowmelt. Engineering works to manage lake volume through two outlets have reduced, but not entirely eliminated, future flood risk.

Climate elasticity of evapotranspiration shifts the water balance of Mediterranean climates during multi-year droughts

Abstract. Multi-year droughts in Mediterranean climates may shift the water balance, that is, the partitioning rule of precipitation across runoff, evapotranspiration, and sub-surface storage. Mechanisms causing these shifts remain largely unknown and are not well represented in hydrologic models. Focusing on measurements from the headwaters of California's Feather River, we found that also in these mixed rain–snow Mediterranean basins a lower fraction of precipitation was partitioned to runoff during multi-year droughts compared to non-drought years. This shift in the precipitation–runoff relationship was larger in the surface-runoff-dominated than subsurface-flow-dominated headwaters (−39 % vs. −18 % decline of runoff, respectively, for a representative precipitation amount). The predictive skill of the Precipitation Runoff Modeling System (PRMS) hydrologic model in these basins decreased during droughts, with evapotranspiration (ET) being the only water-balance component besides runoff for which the drop in predictive skill during drought vs. non-drought years was statistically significant. In particular, the model underestimated the response time required by ET to adjust to interannual climate variability, which we define as climate elasticity of ET. Differences between simulated and data-driven estimates of ET were well correlated with accompanying data-driven estimates of changes in sub-surface storage (ΔS, r=0.78). This correlation points to shifts in precipitation–runoff relationships being evidence of a hysteretic response of the water budget to climate elasticity of ET during and after multi-year droughts. This hysteresis is caused by carryover storage offsetting precipitation deficit during the initial drought period, followed by vegetation mortality when storage is depleted and subsequent post-drought vegetation expansion. Our results point to a general improvement in hydrologic predictions across drought and recovery cycles by including the climate elasticity of ET and better accounting for actual subsurface water storage in not only soil, but also deeper regolith that stores water accessible to roots. This can be done by explicitly parametrizing carryover storage and feedback mechanisms capturing vegetation response to atmospheric demand for moisture.

Assessment of the impact of climate variability and human activities on the runoff in five catchments of the Adriatic Coast of south-central Italy

Study regionSouth-central Italy. Study focusA detailed analysis is carried out in five catchments in the Adriatic Coast where a complex system of artificial lakes, forced conducts and hydropower stations has modified the river regime. This together with the decrease of the annual rainfall in central Italy, make quantifying the relative contributions of human activities and climatic variability to streamflow change a challenging task. Statistically significant trends in annual rainfall and streamflow and change points in runoff have been investigated. Potential changes in precipitation-runoff relationship have been examined and a quantitative evaluation of the effect of climate variability and human activities on runoff has been assessed separately. New hydrological insights for the regionSignificant negative trends in runoff have been found in all the catchments, except the Feltrino river. Precipitation also exhibits negative trend, except the Pescara basin. Significant change points in runoff were detected in all the basins and the study period was divided in a pre-change and a post-change period. By climate elasticity method the climatic variability seems to be the main driving factor of the runoff decrease. Only for the Pescara river human activities were dominant factors of the runoff reduction, as expected from the absence of any significant trend in the annual precipitation. The hydrological model confirms this result, although the assessment of two contributions is affected by some uncertainties.

São Paulo drought: trends in streamflow and their relationship to climate and human-induced change in Cantareira watershed, Southeast Brazil

AbstractThe 2013–2015 drought in the Metropolitan Region of São Paulo exposed the lack of resilience of the regional water supply system, highly dependent on the Cantareira reservoirs. In this paper, inflows to each of the four main Cantareira reservoirs are tested for systematic change. Persistent trends in streamflow, rainfall, temperature and evapotranspiration are first evaluated. Streamflow was also tested for step change. Double-mass curves were employed to assess modification in the precipitation–runoff relationship. Subsequently, we used the climate elasticity method and the ABCD model to quantify the relative contribution of climate and human activities into the detected trends. Only Cachoeira and Atibainha sub-basins showed a significant downward trend in streamflow. The results for step change were also significant, and the year of occurrence coincided with breakpoints in precipitation–runoff relationship. For both Cachoeira and Atibainha, human activities had a more significant impact on streamflow reduction than climate variability. Land use and cover maps suggest that the reduction of pasture/abandoned land parallel to an increase in reforestation/silviculture is behind streamflow reduction. The results highlight the importance of coordinating land-use patterns and water management, as an important contributor beyond any considerations of a changing climate. Implications for better managing regional water resources are discussed.

Simulating the change of precipitation-runoff relationship during drought years in the eastern monsoon region of China

Most hydrological simulation and prediction methods assume that the precipitation-runoff relationship was stationary. However, this assumption was found to be questionable during drought years: the annual runoff coefficients (the ratio of annual runoff to annual precipitation) during drought years tend to be smaller than those during non-drought years. However, little is known about the spatial distribution of the magnitude of runoff coefficient change (RCchange) during drought years, and which factors dominate the spatial pattern of RCchange over a large spatial scale. To answer these questions, this study investigated the RCchange in 265 catchments in China that cover a broad range of climate and landscape conditions. We identified the significant factors affecting RCchange from ten catchment characteristics and developed a multivariate generalized additive model to simulate the spatial pattern of RCchange across the eastern monsoon region of China. Results indicated that the RCchange showed an increasing trend from north to south of China, with values ranging from −67.1% to −0.3%, with the average being −26.4%. The lower RCchange (corresponding to more significant runoff reduction) in drought years was more likely to occur in catchments with dryer climate and lower elevation. The simulated RCchange by the multivariate generalized additive model demonstrated a good agreement with observed RCchange, and the values of Nash-Sutcliffe efficiency between observed and simulated RCchange were 0.77 for training catchments and 0.72 for testing catchments. Finally, we applied the model to extrapolate RCchange to the entire eastern monsoon region of China. The result would benefit water resources management during drought years.

Assessing multidecadal runoff (1970–2010) using regional hydrological modelling under data and water scarcity conditions in Peruvian Pacific catchments

AbstractIn a context of water scarcity in Peruvian Pacific catchments as a crucial issue for Peru, added to the paucity of data availability, we propose a methodology that provides new perspectives for freshwater availability estimation as a base reference for unimpaired conditions. Under those considerations, a regional discharge of 709 m3/s to the Pacific Ocean is estimated with a significant increasing trend of about 43 m3/s per decade over the 1970–2010 period.To represent the multidecadal behaviour of freshwater runoff along the region, a regional runoff analysis is proposed based on hydrological modelling at annual and monthly time step for unimpaired conditions over the whole 1970–2010 period. Differential Split‐Sample Tests are used to assess the hydrological modelling robustness of the GR1A and GR2M conceptual lumped models, showing a satisfactory transposability from dry to wet years inside the thresholds defined for Nash–Sutcliffe and bias criteria. This allowed relating physical catchment characteristics with calibrated and validated model parameters, thus offering a regional perspective for dryland conditions in the study area (e.g., the anticlockwise hysteresis relationship found for seasonal precipitation–runoff relationship) as well as the impacts of climate variability and catchment characteristics.

The variation changes of the precipitation – runoff relationship

The variation of the rainfall-runoff relationship t will lead to the disappearance of the consistency assumption of engineering hydrology, which will influence the planning, design, operation management and development and utilization of water resources. Therefore, the variation diagnosis of the rainfall -runoff relationship has become one of the hot topics and key issues in this area. In this study, the variation points of the precipitation-runoff relationship are defined, the difference between the variation point and the mutation point is distinguished, and the method of variation classification is proposed based on the the dimensionless mean and variation coefficients. Then, a comprehensive diagnosis system of the rainfall-runoff relationship variation is constructed on the basis of systematically summarying and analyzing of the diagnosis method of rainfall - runoff relationship variation at home and abroad. Taking the Weihe River Basin as a case study, the comprehensive diagnosis system is verified by applying it to test the change point the annual runoff time series at the Huaxian hydrological station. And the results show that the comprehensive diagnosis method proposed in this paper is scientific and reasonable.

Investigation and simulations of changes in the relationship of precipitation-runoff in drought years

The frequency and intensity of drought could be increasing with global warming. When drought occurs, the runoff generation process and the precipitation-runoff relationship might change. In this study, the precipitation-runoff relationship in drought years was systematically investigated in the largest freshwater lake basin of China, namely, the Poyang Lake Basin. The results showed that the precipitation-runoff relationship changed significantly (p < 0.05) in drought years. The annual runoff coefficient in drought years decreased by 24.1% compared with that in non-drought years. The decreased magnitude of runoff coefficient had a significant positive relationship (p < 0.01) with the drought severity, i.e., the standardized precipitation index of the year. Two hydrological models calibrated by the objective function Nash-Sutcliffe efficiency significantly overestimated runoff in drought years. Three additional objective functions that are sensitive to low flow simulations significantly improved the accuracy of runoff simulation in drought years. Therefore, we recommend that the selected objective functions for runoff simulation in drought years should be sensitive to low flow simulations. Otherwise, the water resources availability will be overestimated in drought years.

A volumetric water budget of Devils Lake (USA): non-stationary precipitation–runoff relationships in an amplifier terminal lake

ABSTRACTDevils Lake, a terminal lake in eastern North Dakota, rose more than 9 m between 1992 and 2013, producing a 286% increase in lake area, and causing more than US$1 billion in direct damages. An annual volumetric lake water budget is developed from monthly hydroclimatological variables for the period 1951–2010 to investigate the rapid lake expansion. The lake is an amplifier terminal lake in which long-term climatic changes are amplified by positive feedback mechanisms, causing the lake to transition from a precipitation-dominated to a runoff-dominated water budget. Factors specific to the Devils Lake Basin further amplify this positive feedback relationship. These include principles of fill–spill hydrology that operate between individual sub-basins within the closed basin, and between the innumerable wetland complexes within each sub-basin. These factors create a pronounced non-stationary precipitation–runoff relationship in the basin during both long-term wetting and drying phases.

Characterizing drought in terms of changes in the precipitation–runoff relationship: a case study of the Loess Plateau, China

Abstract. The frequency and intensity of drought are increasing dramatically with global warming. However, few studies have characterized drought in terms of its impacts on ecosystem services, the mechanisms through which ecosystems support life. As a result, little is known about the implications of increased drought for resource management. This case study characterizes drought by linking climate anomalies with changes in the precipitation–runoff relationship (PRR) on the Loess Plateau of China, a water-limited region where ongoing revegetation makes drought a major concern. We analyzed drought events with drought durations ≥ 5 years and mean annual precipitation anomaly (PA) values ≤ −5 % during drought periods. The results show that continuous precipitation shifts are able to change the water balance of watersheds in water-limited areas, and multi-year drought events cause the PRR to change with a significantly decreasing trend (p &lt; 0.05) compared to other historical records. For the Loess Plateau as a whole, the average runoff ratio decreased from 10 to 6.8 % during 1991–1999. The joint probability and return period gradually increase with increasing of drought duration and severity. The ecosystem service of water yield is easily affected by drought events with durations equal to or greater than 6 years and drought severity values equal to or greater than 0.55 (precipitation ≤ 212 mm). At the same time, multi-year drought events also lead to significant changes in the leaf area index (LAI). Such studies are essential for ecosystem management in water-limited areas.

The changing hydroclimatology of Southeastern U.S.

The allocation of the water resources of Southeastern United States have given rise to inter-state political conflicts for decades. The limited water resource are not only required to satisfy the demand of rapidly growing urban areas, but also the power industry, recreational needs and the environment. Although water resource variability has been attributed to various teleconnections, knowledge of long-term water resource availability is limited. In this paper historical (1952–2014) precipitation and streamflow data from 18 basins in southeast United States are analyzed for long-term changes in the hydroclimatology, possibly brought by urbanization, climate change and/or changed precipitation patterns. A Pettitt test indicates a change in the precipitation-runoff relationship in the late 1990s. After the shift, basin records indicate decreased precipitation storage and consequently increased streamflow.

Precipitation and runoff variations in the Yellow River Basin of China

Runoff in the Yellow River (YR) of China is steadily declining due to climate change and human activities. In this study, the basic trend and abrupt changes of precipitation at 63 meteorological stations and runoff as measured at six hydrological stations from 1956 to 2010 are analyzed. Results indicate that 38 stations exhibit negative precipitation trends. These stations are mainly located in the lower reaches. All six hydrological stations exhibit declining runoff trends. Abrupt runoff changes were mainly noted in the downstream portion of the basin. These variations then expanded to the middle and upper reaches. A precipitation–runoff double cumulative curve was used to detect the breakpoint of the precipitation–runoff relationship and to identify the impacts of human activities on runoff in the YR. Results show that the relatively uniform precipitation–runoff relationship has changed since 1993 in the upstream reaches and since 1970 in the middle and downstream reaches. Additionally, the relationship was more sensitive in the Lanzhou section. Human activities have become the dominant influencing factor on runoff variation since the 1970s. After the 1990s, the percentages of runoff variations due to human activities were 74.87%, 82.2%, 80.63%, and 88.71% at the Lanzhou, Toudaoguai, Huayuankou, and Lijin stations, respectively.

Hybrid method for assessing the multi-scale periodic characteristics of the precipitation–runoff relationship: a case study in the Weihe River basin, China

Fully elucidating the precipitation–runoff relationship (PRR) is of great significance for better water resources planning and management and understanding hydrological cycle processes. For investigating the multi-scale PRR variability in the Weihe River basin in 1960–2010, a new hybrid method is proposed in which ensemble empirical mode decomposition (EEMD) and cross wavelet transform and wavelet transform coherence are used in combination. With the application of mutual information entropy, monthly precipitation and runoff are decomposed into two parts: high- (HFC) and low-frequency components (LFC). The results show that HFCs are characterized by inter- and intra-annual variations in precipitation and runoff, whereas LFCs display approximately two-year periodicity and contain abundant abnormal information of the raw data. Therefore, the PRR between HFCs exhibited significant correlations at the 95% confidence level over the whole time period. However, the correlations of the PRR between LFCs are not significant for many of the time-frequency domains. Additionally, the phase relations are disordered in these time-frequency domains, and no certain trend in phase angle variations can be identified. Through comparative analysis of the anthropogenic activities and climatic events with PRR variations, it can be concluded that the hybrid method can efficiently capture the PRR in various time-frequency domains.

Hydrological model comparison and assessment: criteria from catchment scales and temporal resolution

ABSTRACTThis study examines the performance of three hydrological models, namely the artificial neural network (ANN) model, the Hydrologiska Byråns Vattenbalansavdelning-D (HBV-D) model, and the Soil and Water Integrated Model (SWIM) over the upper reaches of the Huai River basin. The assessment is done by using databases of different temporal resolution and by further examining the applicability of SWIM for different catchment sizes. The results show that at monthly scale the performance of the ANN model is better than that of HBV-D and SWIM. The ANN model can be applied at any temporal scale as it establishes an artificial precipitation–runoff relationship for various time scales by only using monthly precipitation, temperature and runoff data. However, at daily scale the performance of both HBV-D and SWIM are similar or even better than the ANN model. In addition, the performance of SWIM at a small catchment size (less than 10 000 km2) is much better than at a larger catchment size. In view of climate change modelling, HBV-D and SWIM might be integrated in a dynamical atmosphere-water-cycle modelling rather than the ANN model due to their use of observed physical links instead of artificial relations within a black box.Editor D. Koutsoyiannis; Associate editor D. Hughes