Small Mountainous Catchment Research Articles

Land use and climate change impact on runoff in a small mountainous catchment in Slovakia

The paper focuses on the changes in runoff caused by changes in land use and climate. The study was performed in the Boca River basin, which is located in the Low Tatra National Park in Slovakia. This area has been affected by several severe windstorms in recent decades, which had a significant impact on the changes in forest cover in the Boca River basin occurred in 2004 (Alžbeta) and 2007 (Kyrill and Filip). The bark beetle outbreak followed these windstorms. The first part of the paper focuses on the changes in runoff caused by changes in land use for the period from 1990 to 2018. The design values of short-term rainfalls from actual observations and Corine Land Cover land use are used for the calculation of design floods. The second part focuses on the changes in runoff caused by climate change. The climate change is represented by data from Regional Climate Model (RCM) scenario. The estimation of runoff change is provided for the period 2070 – 2100. These results are compared with the results from actual observations. The design floods are calculated using the Soil Conservation Service - Curve Number method.

Predicting Extreme-Precipitation Effects on the Geomorphology of Small Mountain Catchments: Towards an Improved Understanding of the Consequences for Freshwater Biodiversity and Ecosystems

In 2015 an intense rainfall event hit the Valleys of the Trebbia, Nure, and Aveto watercourses in the Northern Apennines. In about 6 h a mesoscale convective system deployed a stunning amount of precipitation of 340 mm, with an extreme hourly rainfall intensity of >100 mm/h. It triggered debris flows along slopes and stream channels, landslides and floods, which caused serious damages. Through the optimal combination of rainfall data and radar volumes, in this work we present a detailed rainfall analysis, which will serve as a basis to create a quantitative correlation with debris flows over elementary hydrological units. We aim at providing an objective basis for future predictions, starting from the recognition of the forcing meteorological events, and then arriving at the prediction of triggering phenomena and to the debris-flow type. We further provide seven observations/case studies on the effects of extreme-precipitation events on freshwater environments in small mountain catchments. Extreme-precipitation events are becoming more frequent and widespread globally but their ecological effects are still insufficiently understood. In general, the effects of extreme events on inland-waters’ ecosystems are highly context-dependent, ranging from deleterious to beneficial. We therefore highlight the necessity of further studies to characterize these effects in more depth to be able to include appropriate mitigation measures in environmental planning and stewardship.

A soil quality index for evaluation of degradation under land use and soil erosion categories in a small mountainous catchment, Iran

Soil erosion and land use type have long been viewed as being particularly important drivers of soil degradation. The objectives of this study, therefore, were to select a new soil quality index (SQI) which varies significantly with land use/soil erosion, and to evaluate the new SQI using expert opinion. In total, 18 soil physical, chemical, and biochemical properties (indicators) were measured on 56 soil samples collected from four land use/soil erosion categories (rangeland/surface erosion, rangeland/subsurface erosion, cultivated land/surface erosion and dry-farming land/surface erosion). Principal component and classification analysis (PCCA) identified five PCs that explained 77.7% of the variation in soil properties with the biochemical PC varying significantly with land use/soil erosion. General discriminant analysis (GDA) selected urease and clay as the most sensitive properties distinguishing the land use/soil erosion categories. The GDA canonical scores for the new SQI were significantly correlated with expert opinion soil surface summed scores (for soil movement, surface litter, pedestalling, rills and flow pattern) derived using the U.S. Department of the Interior Bureau of Land Management (BLM) method. A forward stepwise general regression model revealed that the new SQI values were explained by soil movement, surface litter, and the summed values of the soil surface factors. Overall, this study confirmed that soil quality in the study area in Iran is controlled by land use and corresponding soil erosion.

Comparative assessment of the flash-flood potential within small mountain catchments using bivariate statistics and their novel hybrid integration with machine learning models

The present study is carried out in the context of the continuous increase, worldwide, of the number of flash-floods phenomena. Also, there is an evident increase of the size of the damages caused by these hazards. Bâsca Chiojdului River Basin is one of the most affected areas in Romania by flash-flood phenomena. Therefore, Flash-Flood Potential Index (FFPI) was defined and calculated across the Bâsca Chiojdului river basin by using one bivariate statistical method (Statistical Index) and its novel ensemble with the following machine learning models: Logistic Regression, Classification and Regression Trees, Multilayer Perceptron, Random Forest and Support Vector Machine and Decision Tree CART. In a first stage, the areas with torrentiality were digitized based on orthophotomaps and field observations. These regions, together with an equal number of non-torrential pixels, were further divided into training surfaces (70%) and validating surfaces (30%). The next step of the analysis consisted of the selection of flash-flood conditioning factors based on the multicollinearity investigation and predictive ability estimation through Information Gain method. Eight factors, from a total of ten flash-floods predictors, were selected in order to be included in the FFPI calculation process. By applying the models represented by Statistical Index and its ensemble with the machine learning algorithms, the weight of each conditioning factor and of each factor class/category in the FFPI equations was established. Once the weight values were derived, the FFPI values across the Bâsca Chiojdului river basin were calculated by overlaying the flash-flood predictors in GIS environment. According to the results obtained, the central part of Bâsca Chiojdului river basin has the highest susceptibility to flash-flood phenomena. Thus, around 30% of the study site has high and very high values of FFPI. The results validation was carried out by applying the Prediction Rate and Success Rate. The methods revealed the fact that the Multilayer Perceptron – Statistical Index (MLP-SI) ensemble has the highest efficiency among the 3 methods.

Assessment of Sediment Transport Functions with the Modified SWAT-Twn Model for a Taiwanese Small Mountainous Watershed

In Taiwan, the steep landscape and highly vulnerable geology make it difficult to predict soil erosion and sediment transportation via variable transport conditions. In this study, we integrated the Taiwan universal soil loss equation (TUSLE) and slope stability conditions in the soil and water assessment tool (SWAT) as the SWAT-Twn model to improve sediment simulation and assess the sediment transport functions in the Chenyulan watershed, a small mountainous catchment. The results showed that the simulation of streamflow was satisfactory for calibration and validation. Before model calibration and validation for sediment, SWAT-Twn with default sediment transport method performed better in sediment simulation than the official SWAT model (version 664). The SWAT-Twn model coupled with the simplified Bagnold equation could estimate sediment export more accurately and significantly reduce the overestimated sediment yield by 65.7%, especially in highly steep areas. Furthermore, five different sediment transport methods (simplified Bagnold equation with/without routing by particle size, Kodoatie equation, Molinas and Wu equation, and Yang sand and gravel equation) were evaluated. It is suggested that modelers who conduct sediment studies in the mountainous watersheds with extreme rainfall conditions should adjust the modified universal soil loss equation (MUSLE) factors and carefully evaluate the sediment transportation equations in SWAT.

Flash Flood Forecasting Using Support Vector Regression Model in a Small Mountainous Catchment

Flash floods in mountainous catchments are often caused by the rainstorm, which may result in more severe consequences than plain area floods due to less timescale and a fast-flowing front of water and debris. Flash flood forecasting is a huge challenge for hydrologists and managers due to its instantaneity, nonlinearity, and dependency. Among different methods of flood forecasting, data-driven models have become increasingly popular in recent years due to their strong ability to simulate nonlinear hydrological processes. This study proposed a Support Vector Regression (SVR) model, which is a powerful artificial intelligence-based model originated from statistical learning theory, to forecast flash floods at different lead times in a small mountainous catchment. The lagged average rainfall and runoff are identified as model input variables, and the time lags associated with the model input variables are determined by the hydrological concept of the time of response. There are 69 flash flood events collected from 1984 to 2012 in a mountainous catchment in China and then used for the model training and testing. The contribution of the runoff variables to the predictions and the phase lag of model outputs are analyzed. The results show that: (i) the SVR model has satisfactory predictive performances for one to three-hours ahead forecasting; (ii) the lagged runoff variables have a more significant effect on the predictions than the rainfall variables; and (iii) the phase lag (time difference) of prediction results significantly exists in both two- and three-hours-ahead forecasting models, however, the input rainfall information can assist in mitigating the phase lag of peak flow.

Estimation of macropore flow characteristics in stony soils of a small mountain catchment

The rapid movement of water in a complex system of preferential pathways in soils is a common natural phenomenon. However, quantification of the properties of this phenomenon is not easy and such results are not often found in the literature. The paper presents results of quantification of the number of macropores, their relative volume and the ratio of water infiltrating through the macropores for five study sites with stony soils located in a mountain catchment of northern Slovakia. These characteristics were calculated from the saturated and unsaturated soil hydraulic conductivities under the assumptions of the Poiseuille’s law. Saturated hydraulic conductivities were estimated by single ring infiltrometer measurements in the field and by falling head method in the laboratory. Unsaturated hydraulic conductivities were estimated by tension infiltration measurements in the field (using a minidisc infiltrometer, Decagon Devices). The largest amount of macropores (defined as the pores with radius larger than 1.5 mm) in the entire soil profile (7–5276 per 1 m2) was found at forest sites overgrown by spruce with high to extremely high stoniness. The number of macropores decreased with depth of the soil profile at all study sites. The contribution of macropore flow to water infiltration varied from approximately 63% to almost 100%. Although macropores can conduct a large proportion of water, their relative volume is small (0.001–0.9%). The relative volume of all macropores (active and non-active) estimated from the soil porosity and the water retention function was in the range 2–23%.

Continuous Measurement of Surface Temperature using UAV in Small Bare Mountainous Catchment and Comparison with Radiation and Soil Water Content

Continuous Measurement of Surface Temperature using UAV in Small Bare Mountainous Catchment and Comparison with Radiation and Soil Water Content

Generalization of the average annual water runoff of the rivers according to the hydrographic zoning of Ukraine

Aim of the research is to estimate average annual water runoff of rivers of Ukraine within river basins and sub-basins respectively to the Hydrographic zoning of Ukraine. The territory of Ukraine is divided into hydrographic units - nine areas of river basins and nine sub-basins. Methods of the research - methods of mathematical statistics and statistical analysis of the relations between hydrological variables. Scientific novelty of the research is to process the base of mean annual water discharge of rivers of Ukraine from 294 hydrometric posts from the beginning of observations until 2015 inclusive. 83 % of the posts have observation periods ≥ 50 years, and only 17 % have ≤ 50 years. Also, for the first time, the generalization of the mean annual water runoff in the rivers was carried out according to the scheme of Hydrographic zoning of Ukraine. Рractical value is determined by further development of studies of the average annual runoff of the rivers in Ukraine and its generalization according to modern observation data. Research results. Analysis of the relative values of the mean standard deviation in the calculations of the mean annual runoff of the rivers of Ukraine has shown that the series of its observations are considered representative. Dependences of the average annual discharges of water in the rivers from the area of their catchments for river basins and sub-basins proved to be of rather high approximation. Correlations is from 0,76 to 0,99, which gives grounds for using them in practical calculations within the studied river basins and sub-basins. Analysis of the spatial changes of the specific discharge of rivers in the territory of Ukraine showed that their largest values are (34-39 l · s - 1 · km - 2) fixed on small mountain catchments of the Carpathian and Crimea. In plains they vary from 12,6 l · s - 1 · km - 2 in the river basin of the Vistula River to 0,1-0,2 l · s - 1 · km - 2 in the rivers basin of the Black Sea. The general tendency is their decrease from the northwest to the southeast and south. The variation coefficients of the the mean annual runoff for rivers in the flat part of Ukraine are within the range of 0,23÷0,65, for the mountain basins 0,20÷0,45. Coefficients of asymmetry have a low positive value for the entire territory of Ukraine. The generalized ratio between coefficients of asymmetry and variation is 2÷2,5.

Inter-Comparison of Rain-Gauge, Radar, and Satellite (IMERG GPM) Precipitation Estimates Performance for Rainfall-Runoff Modeling in a Mountainous Catchment in Poland

Precipitation is one of the essential variables in rainfall-runoff modeling. For hydrological purposes, the most commonly used data sources of precipitation are rain gauges and weather radars. Recently, multi-satellite precipitation estimates have gained importance thanks to the emergence of Integrated Multisatellite Retrievals for Global Precipitation Measurement (IMERG GPM), a successor of a very successful Tropical Rainfall Measuring Mission (TRMM) mission which has been providing high-quality precipitation estimates for almost two decades. Hydrological modeling of mountainous catchment requires reliable precipitation inputs in both time and space as the hydrological response of such a catchment is very quick. This paper presents an inter-comparison of event-based rainfall-runoff simulations using precipitation data originating from three different sources. For semi-distributed modeling of discharge in the mountainous river, the Hydrologic Engineering Center-Hydrologic Modelling System (HEC-HMS) is applied. The model was calibrated and validated for the period 2014–2016 using measurement data from the Upper Skawa catchment a small mountainous catchment in southern Poland. The performance of the model was assessed using the Nash–Sutcliffe efficiency coefficient (NSE), Pearson’s correlation coefficient (r), Percent bias (PBias) and Relative peak flow difference (rPFD). The results show that for the event-based modeling adjusted radar rainfall estimates and IMERG GPM satellite precipitation estimates are the most reliable precipitation data sources. For each source of the precipitation data the model was calibrated separately as the spatial and temporal distributions of rainfall significantly impact the estimated values of model parameters. It has been found that the applied Soil Conservation Service (SCS) Curve Number loss method performs best for flood events having a unimodal time distribution. The analysis of the simulation time-steps indicates that time aggregation of precipitation data from 1 to 2 h (not exceeding the response time of the catchment) provide a significant improvement of flow simulation results for all the models while further aggregation, up to 4 h, seems to be valuable only for model based on rain gauge precipitation data.

Isotopic response of run‐off to forest disturbance in small mountain catchments

AbstractStable water isotopes were applied to trace hydrological processes in an undisturbed (mature spruce forest) and a nearby disturbed (deforested from a bark beetle outbreak) lake catchments in the Czech Republic. Both catchments are situated above 1,000 m a.s.l. within the Šumava National Park and have similar environmental conditions. The isotopic compositions of precipitation, creeks, springs, and lakes were sampled at 3‐week intervals over one hydrological year. Water inputs to catchments were derived from isotopically similar local precipitation, whereas run‐off was found to have different isotopic signatures. Creeks in the undisturbed catchment had ~1‰ and ~7‰ higher δ18O and δ2H with ~2‰ lower d‐excess than in the disturbed catchment. The d‐excess in creeks of the undisturbed catchment was more pronounced, particularly during snowmelt, and highly heterogeneous as compared with the disturbed catchment. Creeks in the undisturbed catchment were mainly fed by precipitation during the warm period (May–October), whereas creeks in the disturbed catchment were mostly fed by precipitation during the cold period (November–April). Estimated mean transit times of creeks and springs were ~6 months, except for two creeks in the undisturbed catchment, which had residence times of ~1 year. Although evaporation and transpiration fluxes were apparently reduced in the disturbed catchment, transpiration ratios were similar for both catchments. The difference in isotope signatures between catchments was attributed to the altered role of the forest canopy in temporal water distribution, which produced changes in the water cycle, potentially influencing important biogeochemical processes.

Impact of Rain Gauges Distribution on the Runoff Simulation of a Small Mountain Catchment in Southern Ecuador

In places with high spatiotemporal rainfall variability, such as mountain regions, input data could be a large source of uncertainty in hydrological modeling. Here we evaluate the impact of rainfall estimation on runoff modeling in a small páramo catchment located in the Zhurucay Ecohydrological Observatory (7.53 km2) in the Ecuadorian Andes, using a network of 12 rain gauges. First, the HBV-light semidistributed model was analyzed in order to select the best model structure to represent the observed runoff and its subflow components. Then, we developed six rainfall monitoring scenarios to evaluate the impact of spatial rainfall estimation in model performance and parameters. Finally, we explored how a model calibrated with far-from-perfect rainfall estimation would perform using new improved rainfall data. Results show that while all model structures were able to represent the overall runoff, the standard model structure outperformed the others for simulating subflow components. Model performance (NSeff) was improved by increasing the quality of spatial rainfall estimation from 0.31 to 0.80 and from 0.14 to 0.73 for calibration and validation period, respectively. Finally, improved rainfall data enhanced the runoff simulation from a model calibrated with scarce rainfall data (NSeff 0.14) from 0.49 to 0.60. These results confirm that in mountain regions model uncertainty is highly related to spatial rainfall and, therefore, to the number and location of rain gauges.

Isotopic hydrograph separation in two small mountain catchments during multiple events

Two-component isotopic hydrograph separation (IHS) was developed to determine the event- and pre-event components of a single storm event. Its application for several sucessive events requires repeated determination of isotopic signatures of end-members (precipitation, pre-event component) for each event. The existence of several possible alternative signatures results in differences in calculated contributions of event-/pre- event components. This article addresses the question of how big the differences could be in small mountain catchments with different methods for detemining the end member signatures. We analyzed data on isotopic composition of daily/event precipitation at different elevations in two catchments located in the highest part of the Carpathians in July 2014.The isotopic composition of streamflow sampled every 4-6 hours was analyzed as well. Elevational gradients of δ18O and δ2H in precipitation in the study period were -0.18 ‰ 100 m-1 and -1.1 ‰ 100 m-1, respectively. An elevation gradient in deuterium excess (0.29 ‰ 100 m-1) was also found. Precipitation on the windward side of the mountains was isotopically lighter than expected for a given rain gauge elevation. Five large rainfall-runoff events occurred in the study period in the meso-scale catchment of the Jalovecký creek (Western Tatra Mountains, area 22.2 km2) and in the headwater catchment of the Škaredý creek (High Tatra Mountains, area 1.4 km2). Isotopic hydrograph separation was conducted using eight options for the isotopic signatures of event and pre-event water. The isotopic signature of the event water (rainfall) was alternatively represented by data from high or low elevations. Pre-event water was represented either by the streamflow before the event or by the value taken from the statistics of the long-term data on isotopic composition of the stream. Both isotopes (18O and 2H) were used to calculate event water fractions during peak flows of individual events. Calculated peak flow event water fractions were below 0.2-0.3 for most events. However, the differences in calculated event water fractions for alternative isotopic composition of end-members were significant even if we did not take into account changes in isotopic composition during individual rainfalls. Coefficients of variation for event water fractions calculated for various options varied during individual events from 0.14 to 0.36. It is therefore perhaps better to use a range of possible values instead of a single accurate number to interpret the IHS results. Hydrograph separations based on 18O and 2H provided similar results.

Direct response of small non-glaciated headwater catchments to Late Quaternary climate change: The Valle de la Fueva, southern Pyrenees

Topographic depressions in mountain regions function as sediment traps and their morpho-sedimentary archives provide important insight into erosion and sediment dynamics in mountain ranges. In order to establish the (direct) drivers and timing of erosion phases in the southern central Pyrenees, small non-glaciated headwater catchments in the highly erodible Valle de la Fueva drainage basin were investigated as to their morphogenetic development.Geomorphic mapping revealed three morphogenetic complexes associated with the Valle de la Fueva drainage basin development: (i) high-lying talus plateaus and lateral slopes, (ii) extensive glacis and pedimentation surfaces, and (iii) deeply incised ravines with late-stage fluvial-cut terraces. The talus slopes are the oldest preserved geomorphic units of the Valle de la Fueva and potentially denote the initial drainage basin topography. Talus bedrock and boulder exposure ages cluster yet between 21 and 40 10Be-ka, indicating that the carbonate-dominated talus surfaces were rejuvenated during the last glacial cycle (MIS 4–MIS 2). The younger morphogenetic complexes (ii, iii) point to successive basin erosion stages from diffusive surface pedimentation under cold climate conditions (ii) to rapid linear fluvial dissection and late-stage terrace formation (iii). Denoting the abandonment of pediments as actively eroding surfaces, 10Be depth profiles of glacis cover sediments yielded consistent late MIS 2 exposure ages. The abandoning of pediments marks the onset of rapid fluvial downcutting of deep ravines (iii) around the Holocene climate transition, which involved substantial sediment routing and basin scale erosion. Lower fluvial-cut terraces denote the termination of intense catchment erosion at around 14–8–10Be-ka and minor fluvial incision during Middle-Late Holocene.This research shows that small non-glaciated mountain catchments feature valuable high-resolution morpho-sedimentary archives that are useful for assessing the direct impact of major climate transitions on surface processes and landscape transformation in mountain regions. Improvements in quantifying the timing and rates of (paleo-) surface processes are vital for enhancing regional glaciofluvial chronologies and large-scale basin erosion models that are possibly afflicted with time-averaging biases.

Differences in N loading affect DOM dynamics during typhoon events in a forested mountainous catchment

The dissolved organic matter (DOM) and nutrient dynamics in small mountainous rivers (SMRs) strongly depend on hydrologic conditions, and especially on extreme events. Here, we investigated the quantity and quality of DOM and inorganic nutrients during base-flow and typhoon events, in a chronically N-saturated mainstream and low N-loaded tributaries of a forested small mountainous reservoir catchment in Taiwan. Our results suggest that divergent transport mechanisms were triggered in the mainstream vs. tributaries during typhoons. The mainstream DON increased from 3.4 to 34.7% of the TDN pool with a static DOC:NO3-N ratio and enhanced DOM freshness, signalling a N-enriched DOM transport. Conversely, DON decreased from 46 to 6% of the TDN pool in the tributaries and was coupled with a rapid increase of the DOC:NO3-N ratio and humified DOM signals, suggesting the DON and DOC were passively and simultaneously transported. This study confirmed hydrology and spatial dimensions being the main drivers shaping the composition and concentration of DOM and inorganic nutrients in small mountainous catchments subject to hydrologic extremes. We highlighted that the dominant flow paths largely controlled the N-saturation status and DOM composition within each sub-catchment, the effect of land-use could therefore be obscured. Furthermore, N-saturation status and DOM composition are not only a result of hydrologic dynamics, but potential agents modifying the transport mechanism of solutes export from fluvial systems. We emphasize the importance of viewing elemental dynamics from the perspective of a terrestrial-aquatic continuum; and of taking hydrologic phases and individual catchment characteristics into account in water quality management.

Analysis of sediment from steps in a small catchment in the Polish Carpathians in relation to the transition zone between the hillslope and fluvial system

An investigation to determine the transition zone between the hillslope system and the fluvial system in connection with the degree of differentiation of sediment located upstream and downstream of log steps (LS) and boulder steps (BS) was undertaken in the channel of a stream located in a small mountain catchment area in the Silesian Beskidy Mts. (Polish Carpathians), a stream channel into which large wood was introduced artificially several years ago. Granulometric analyses of 41 samples taken from upstream (21) and downstream (20) of steps were performed to analyze the effectiveness of log steps and boulder steps in the differentiation of mineral sediment.It was demonstrated that log steps have a significant impact on the functioning of the channel by shaping its longitudinal profile and affecting the transport of mineral material, whereas boulder steps retain less sediment, showing the beneficial effects of artificial creation of log steps in mountain stream channels. Results of granulometric analyses suggested the location of the theoretical transition zone between the hillslope system and the fluvial system, suggesting the location and range of this zone in small mountain catchment areas can change depending on the intensity and range of slope processes in the longitudinal profile of the channel.

Assessment of Small Mountainous Catchment Runoff at Yongdam-dam Guryang

Assessment of Small Mountainous Catchment Runoff at Yongdam-dam Guryang

Impact of land-cover change related to urbanization on surface runoff estimation

Civil engineering structures are highly responsible for the negative aspects of urbanization on the hydrological cycle. Due to the land cover change and an increase of impervious areas the risk of flooding can rise significantly. The paper presents the impact of urbanization processes on surface runoff estimation for a small mountainous catchment between 1990-2012.

Natural and anthropogenic agents in the present geomorphological dynamics of the Torrent Corlo mountain catchment (Northern Apennines, Italy)

The morphological changes affecting a small mountain catchment in the territory of Modena (Italy) have been taken into account. In particular, based on detailed field surveys and analysis of remote sensing imagery, an assessment was made on the natural and anthropogenic agents which have modelled this basin in the past 80 years. For several decades, the Torrent Corlo catchment was subject to intense exploitation of clay raw materials in order to supply a flourishing ceramic tile industry. As a result, a strong, negative impact has affected the landscape and, in some areas, the geomorphological evolution due to man’s activities greatly surpasses the changes ascribable to natural processes, such as landslides or stream channel processes. Since 2008, all quarrying activities have ceased and the local public administration has started a project of reclamation of the abandoned clay pits, although it will not be possible to restore completely the original, natural features of this catchment. A comparison is made with the sub-catchment Rio delle Vigne valley, which has been spared from anthropogenic changes and has been affected only by natural gravity-induced processes, which have produced some twelve landslides. The final products of this research are the implementation of the first geomorphological map of the T. Corlo mountain catchment and an updated version of the hydrological hazard map, in which active and dormant landslides are much more numerous than those mapped in any other previous geological map (s.l.). These documents can be utilized for any intervention by public boards aiming at recuperating and improving this intensely exploited area.

Modeling flash floods in ungauged mountain catchments of China: A decision tree learning approach for parameter regionalization

Flash floods in small mountain catchments are one of the most frequent causes of loss of life and property from natural hazards in China. Hydrological models can be a useful tool for the anticipation of these events and the issuing of timely warnings. One of the main challenges of setting up such a system is finding appropriate model parameter values for ungauged catchments. Previous studies have shown that the transfer of parameter sets from hydrologically similar gauged catchments is one of the best performing regionalization methods. However, a remaining key issue is the identification of suitable descriptors of similarity. In this study, we use decision tree learning to explore parameter set transferability in the full space of catchment descriptors. For this purpose, a semi-distributed rainfall-runoff model is set up for 35 catchments in ten Chinese provinces. Hourly runoff data from in total 858 storm events are used to calibrate the model and to evaluate the performance of parameter set transfers between catchments. We then present a novel technique that uses the splitting rules of classification and regression trees (CART) for finding suitable donor catchments for ungauged target catchments. The ability of the model to detect flood events in assumed ungauged catchments is evaluated in series of leave-one-out tests. We show that CART analysis increases the probability of detection of 10-year flood events in comparison to a conventional measure of physiographic-climatic similarity by up to 20%. Decision tree learning can outperform other regionalization approaches because it generates rules that optimally consider spatial proximity and physical similarity. Spatial proximity can be used as a selection criteria but is skipped in the case where no similar gauged catchments are in the vicinity. We conclude that the CART regionalization concept is particularly suitable for implementation in sparsely gauged and topographically complex environments where a proximity-based regionalization concept is not applicable.