Standard normal homogeneity test as a tool to detect change points in climate-related river discharge variation: case study of the Kupa River Basin

ABSTRACT

The study addresses homogeneity testing of annual discharge time series for eight hydrological stations and five annual climate time series for one weather station in the Kupa River Basin, between Slovenia and Croatia, and global annual average surface temperature time series for the period 1961–2010. The standard normal homogeneity test (SNHT) was used to detect both abrupt and gradual linear trend homogeneity breaks. The results reveal natural change points at the beginning of the 1980s. Absolute homogeneity testing of average annual weather station-level air pressure, annual precipitation, differences between precipitation totals and potential evapotranspiration and surface runoff from the independent observation time series confirmed an abrupt shift, also at the beginning of the 1980s. The trend of local air temperature for 1985–2000, which partly coincides with global surface temperature trend for 1974–2005, strengthened the river discharge regime shift since the beginning of the 1980s. These results could improve climate variation monitoring and estimation of the impact of climate variation on the environment in the area. Generally, an indication of climate regime change points and an assessment of their duration could provide significant benefits for the society.     

Shift trend and step changes for runoff time series in the Shiyang River basin,northwest China

Shift trend and step changes were detected for runoff time series in the Shiyang River basin, one of the inland river basins in north‐west China. Annual runoff data from eight tributaries as well as both annual and monthly runoff from the mainstream from 1958 to 2003 were used. Seven statistical test methods were employed to identify the shift trends and step changes in the study. Mann–Kendall test, Spearman's Rho test, linear regression and Hurst exponent were used to detect past and future shift trends for runoff time series, while the distributed‐free CUSUM test, cumulative deviations and the Worsley likelihood ratio test were used to detect step changes for the same time series. Results showed that the annual runoff from Zamu, Huangyang and Gulang rivers, as well as both annual and monthly runoff from the mainstream, show statistically significant decreasing trends. Future tendency of runoff for both tributaries and mainstream were consistent with that from 1958 to 2003. Step changes probably occurred in 1961 for the runoff from Huangyang, Gulang and Dajing rivers according to the Worsley likelihood ratio test, but no similar results were found using the other two test methods. Three change points (1979, 1974 and 1973) were detected for the mainstream using different methods. These change points were close to the years that reservoirs started to be operated. Both climate change and human activities, especially the latter, contributed to the decreasing runoff in the study area. Between 21% and 79% of the reduction in runoff from the mainstream was due to the impact of human activities during the past few decades. Copyright © 2008 John Wiley & Sons, Ltd.     

Hydrological trends and the evolution of catchment research in the Alptal valley,central Switzerland

When the observation of small headwater catchments in the pre-Alpine Alptal valley (central Switzerland) started in the late 1960s, the researchers were mainly interested in questions related to floods and forest management. Investigations of geomorphological processes in the steep torrent channels followed in the 1980s, along with detailed observations of biogeochemical and ecohydrological processes in individual forest stands. More recently, research in the Alptal has addressed the impacts of climate change on water supply and runoff generation. In this article, we describe, for the first time, the evolution of catchment research at Alptal, and present new analyses of long-term trends and short-term hydrologic behaviour. Hydrometeorological time series from the past 50 years show substantial interannual variability, but only minimal long-term trends, except for the ~2°C increase in mean annual air temperature over the 50-year period, and a corresponding shift towards earlier snowmelt. Similar to previous studies in larger Alpine catchments, the decadal variations in mean annual runoff in Alptal's small research catchments reflect the long-term variability in annual precipitation. In the Alptal valley, the most evident hydrological trends were observed in late spring and are related to the substantial change in the duration of the snow cover. Streamflow and water quality are highly variable within and between hydrological events, suggesting rapid shifts in flow pathways and mixing, as well as changing connectivity of runoff-generating areas. This overview illustrates how catchment research in the Alptal has evolved in response to changing societal concerns and emerging scientific questions.     

Spatial and temporal characteristics of changes in precipitation during 1957�C2007 in the Haihe River basin, China

The present study explores the spatial and temporal changing patterns of the precipitation in the Haihe River basin of North China during 1957–2007 at annual, seasonal and monthly scales. The Mann–Kendall and Sen’s T tests are employed to detect the trends, and the segmented regression is applied to investigate possible change points. Meanwhile, Sen’s slope estimator is computed to represent the magnitudes of the temporal trends. The regional precipitation trends are also discussed based on the regional index series of four sub-basins in the basin. Serial correlation of the precipitation series is checked prior to the application of the statistical test to ensure the validity of trend detection. Moreover, moisture flux variations based on the NCEP/NCAR reanalysis dataset are investigated to further reveal the possible causes behind the changes in precipitation. The results show that: (1) Although the directions of annual precipitation trends at all stations are downward, only seven stations have significant trends at the 90% confidence level, and these stations are mainly located in the western and southeastern Haihe River basin. (2) Summer is the only season showing a strong downward trend. For the monthly series, significant decreasing trends are mainly found during July, August and November, while significant increasing trends are mostly observed during May and December. In comparison with the annual series, more intensive changes can be found in the monthly series, which may indicate a shift in the precipitation regime. (3) Most shifts from increasing trends to decreasing trends occurred in May–June, July, August and December series, while opposed shifts mainly occurred in November. Summer is the only season displaying strong shift trends and the change points mostly emerged during the late 1970s to early 1980s. (4) An obvious decrease in moisture flux is observed after 1980 in comparison with the observations before 1980. The results of similar changing patterns between monthly moisture budget and precipitation confirmed that large-scale atmospheric circulation may be responsible for the shift in the annual cycle of precipitation in the Haihe River basin. These findings are expected to contribute to providing more accurate results of regional changing precipitation patterns and understanding the underlying linkages between climate change and alterations of hydrological cycles in the Haihe River basin.     

Quantitative assessment of the impact of climate variability and human activities on runoff changes for the upper reaches of Weihe River

In the wake of global and regional climate change and heightened human activities, runoff from some rivers in the world, especially in the arid and semi-arid regions, has significantly decreased. To reveal the varying characteristics leading to the change in runoff, detecting the influencing factors has been important in recent scientific discussions for water resources management in drainage basins. In this paper, an investigation into attributing the runoff response to climate change and human activities were conducted in two catchments (Wushan and Shetang), situated in the upper reaches of Weihe River in China. Prior to the identification of the factors that influenced runoff changes, the Mann–Kendall test was adopted to identify the trends in hydro-climate series. Also, change-points in the annual runoff were detected through Pettitt’s test and the precipitation–runoff double cumulative curve method. It is found that both catchments presented significant negative trend in annual runoff and the detected change-point in runoff occurs in 1993. Hence, the pre-change period and post-change period are defined before and after 1993, respectively. Then, runoff response to climate change and human activities was quantitatively evaluated on the basis of hydrologic sensitivity analysis and hydrologic model simulation. They provided similar estimates of the percentage change in mean annual runoff for the post-change period over the considered catchments. It is found that the decline in annual runoff over both catchments can be mainly attributed to the human activities, the reduction percentages due to human activities range from 59 to 77 %. The results of this study can provide a reference for the development, utilization and management of the regional water resources and ecological environment protection.     

Long‐term streamflow trends in the middle reaches of the Yellow River Basin: detecting drivers of change

The catchments in the Loess Plateau, in China's middle reaches of the Yellow River Basin, experienced unprecedented land use changes in the last 50 years as a result of large‐scale soil conservation measure to control soil erosion. The climate of the region also exhibited some levels of change with decreased precipitation and increased temperature. This study combined the time‐trend analysis method with a sensitivity‐based approach and found that annual streamflow in the Loess Plateau decreased significantly since the 1950s and surface runoff trends appear to dominate the streamflow trends in most of the catchments. Annual baseflow exhibited mostly downward trends, but significant upward trends were also observed in 3 out of 38 gauging stations. Mean annual streamflow during 1979?2010 decreased by up to 65% across the catchments compared with the period of 1957?1978, indicating significant changes in the hydrological regime of the Loess Plateau. It is estimated that 70% of the streamflow reduction can be attributed to land use change, while the remaining 30% is associated with climate variability. Land use change because of the soil conservation measures and reduction in precipitation are the key drivers for the observed streamflow trends. These findings are consistent with results of previous studies for the region and appear to be reasonable given the accelerated level of the soil conservation measures implemented since the late 1970s. Changes in sea surface temperature in the Pacific Ocean, as indicated by variations in El Niño–Southern Oscillation and phase shifts of the Pacific Decadal Oscillation, appear to have also affected the annual streamflow trends. The framework described in this study shows promising results for quantifying the effects of land use change and climate variability on mean annual streamflow of catchments within the Loess Plateau. Copyright © 2015 John Wiley & Sons, Ltd.     

Impacts of climate change and land use change on runoff of forest catchment in northeast China

Hydrological processes change from the impacts of climate variability and human activities. Runoff in the upper reaches of the Hun‐Taizi River basin, which is mainly covered by forests in northeast China, decreased from 1960 to 2006. The data used in this study were based on runoff records from six hydrological stations in the upper reaches of the Hun‐Taizi River basin. Nonparametric Mann–Kendall statistic was used to identify change trends and abrupt change points and consequently analyze the change characteristics in hydrological processes. The abrupt change in the annual runoff in most subcatchments appeared after 1975. Finally, the effects of climate change and land cover change on water resources were identified using regression analysis and a hydrology model. Results of the regression analysis suggest that the correlation coefficients between precipitation and runoff prior to the abrupt change were higher compared with those after the abrupt change. Moreover, using hydrology model analysis, the water yield was found to increase because of the decrease in forest land. Copyright © 2012 John Wiley & Sons, Ltd.     

Direct evidence of the feedback between climate and weathering

Long-term climate moderation is commonly attributed to chemical weathering; the higher the temperature and precipitation the faster the weathering rate. Weathering releases divalent cations to the ocean via riverine transport where they promote the drawdown of CO₂ from the atmosphere by the precipitation and subsequent burial of carbonate minerals. To test this widely-held hypothesis, we performed a field study determining the weathering rates of 8 nearly pristine north-eastern Iceland river catchments with varying glacial cover over 44 years. The mean annual temperature and annual precipitation of these catchments varied by 3.2 to 4.5 °C and 80 to 530%, respectively during the study period. Statistically significant linear positive correlations were found between mean annual temperature and chemical weathering in all 8 catchments and between mean annual temperature and both mechanical weathering and runoff in 7 of the 8 catchments. For each degree of temperature increase, the runoff, mechanical weathering flux, and chemical weathering fluxes in these catchments are found to increase from 6 to 16%, 8 to 30%, and 4 to 14% respectively, depending on the catchment. In contrast, annual precipitation is less related to the measured fluxes; statistically significant correlations between annual precipitation and runoff, mechanical weathering, and chemical weathering were found for 3 of the least glaciated catchments. Mechanical and chemical weathering increased with time in all catchments over the 44 year period. These correlations were statistically significant for only 2 of the 8 catchments due to scatter in corresponding annual runoff and average annual temperature versus time plots. Taken together, these results 1) demonstrate a significant feedback between climate and Earth surface weathering, and 2) suggest that weathering rates are currently increasing with time due to global warming.     

Response of streamflow to climate change and human activity in Xitiaoxi river basin in China

In recent years, the Xitiaoxi river basin in China has experienced intensified human activity, including city expansion and increased water demand. Climate change also has influenced streamflow. Assessing the impact of climate variability and human activity on hydrological processes is important for water resources planning and management and for the sustainable development of eco‐environmental systems. The non‐parametric Mann–Kendall test was employed to detect the trends of climatic and hydrological variables. The Mann–Kendall–Sneyers test and the moving t‐test were used to locate any abrupt change of annual streamflow. A runoff model, driven by precipitation and potential evapotranspiration, was employed to assess the impact of climate change on streamflow. A significant downward trend was detected for annual streamflow from 1975 to 2009, and an abrupt change occurred in 1999, which was consistent with the change detected by the double mass curve test between streamflow and precipitation. The annual precipitation decreased slightly, but upward trends of annual mean temperature and potential evapotranspiration were significant. The annual streamflow during the period 1999–2009 decreased by 26.19% compared with the reference stage, 1975–1998. Climate change was estimated to be responsible for 42.8% of the total reduction in annual streamflow, and human activity accounted for 57.2%. Copyright © 2012 John Wiley & Sons, Ltd.     

Hydro-climatic trends in the Abay/Upper Blue Nile basin,Ethiopia

Trends of the three hydro-meteorological variables precipitation, temperature and stream flow, represented by 13, 12, and 9 gauging stations, respectively, within the Abay/Upper Blue Nile basin have been studied to support water management in the region. The Trends were evaluated over different time periods depending on data availability at the stations. The statistical Mann–Kendall and Pettitt tests have been used to assess trends and change points respectively. The tests have been applied to mean annual, monthly, seasonal, 1- and 7-days annual minimum and maximum values for streamflow, while mean annual, monthly and seasonal timescales were applied to meteorological variables. The results are heterogeneous and depict statistically significant increasing/decreasing trends. Besides, it showed significant abrupt change of point upward/downward shift for streamflow and temperature time series. However, precipitation time series did not show any statistically significant trends in mean annual and seasonal scales across the examined stations.Increasing trends in temperature at different weather stations for the mean annual, rainy, dry and small rainy seasons are apparent. The mean temperature at Bahir Dar – typical station in the Lake Tana sub basin, has been increasing at the rate of about 0.5 °C/decade, 0.3 °C/decade in rainy season (June–September), 0.6 °C/decade in small rainy season (March–May), and 0.6 °C/decade in dry season (October–February). Other stations in the Abay/Upper Blue Nile show comparable results. Overall it is found that trends and change point times varied considerably across the stations and catchment to catchment. Identified significant trends can help to make better planning decisions for water management. However, the cause attributes to the observed changes in hydro-meteorological variables need further research. In particular the combined effects of land use/land cover change and climate variability on streamflow of Abay/Blue Nile basin and its tributaries needs to be understood better.     

The impact of climatic variability and change in the hydroclimatology of Lake Winnipeg watershed

The hydroclimatology of prairie‐dominated portions of the Lake Winnipeg watershed was investigated to determine the possible presence of trends and shifts in variables that may influence the streamflow regimes and water quality of Lake Winnipeg. The total annual streamflow, precipitation, runoff ratio and daily maximum streamflow in the two major tributaries of the Assiniboine River and Red River were analysed for a range of nonstationary behaviours. Each of these rivers has been gauged for more than 90 years. The methods used included a nonparametric Mann–Kendall test modified to account for diverse memory properties (i.e. short term versus long term) and a Bayesian change point detection model to identify possible segments of time series with inconsistent nonstationary behaviour. Although there is no evidence of statistically significant trends in precipitation and streamflow in the Assiniboine River watershed, a shift‐type nonstationarity in annual runoff and runoff ratio was observed in this area, which is manifested in the form of a sequence of wet and dry spells during the last century. Precipitation and runoff metrics in the American portion of the study area (i.e. Red River watershed) were characterised with both gradual and abrupt changes with an extremely increasing rate of streamflow beyond that of intensified precipitation. The nonproportional watershed runoff response is attributed to the dynamic nature of contributing areas that, together with the semiarid climate, leads to sudden changes of streamflow due to major or even some times minor changes in climate inputs. It is evident that streamflow in the depression‐dominated landscapes of the semiarid glaciated plains of North America is particularly sensitive and vulnerable to minor climate variability and change. This study provides valuable insights into the highly complex precipitation–runoff relationship in depression‐dominated landscapes and could have important implications for water management in this part of North America and comparable regions. Copyright © 2011 John Wiley & Sons, Ltd.     

The impact of climate change on runoff in the Yarlung Tsangpo River basin in the Tibetan Plateau

The Tibetan Plateau (TP) is the “water tower of Asia” and it plays a key role on both hydrology and climate for southern and eastern Asia. It is critical to explore the impact of climate change on runoff for better water resources management in the TP. However, few studies pay attention to the runoff response to climate change in large river systems on the TP, especially in data-sparse upstream area. To complement the current body of work, this study uses two rainfall-runoff models (SIMHYD and GR4J) to simulate the monthly and annual runoff in the upstream catchments of the Yarlung Tsangpo River basin (YTR) under historical (1962–2002) and future (2046–2065 A1B scenario) climate conditions. The future climate series are downscaled from a global climate model (MIROC3.2_hires) by a high resolution regional climate model (RegCM3). The two rainfall-runoff models successfully simulate the historical runoff for the eight catchments in the YTR basin, with median monthly runoff Nash–Sutcliffe Efficiency of 0.86 for SIMHYD and 0.83 for GR4J. The mean annual future temperature in eight catchments show significant increase with the median of +3.8 °C. However, the mean annual future precipitation shows decrease with the median of ?5.8 % except in Lhatse (+2.0 %). The two models show similar modeling results that the mean annual future runoff in most of catchments (seven in eight) shows decrease with the median of ?13.9 % from SIMHYD and ?15.2 % from GR4J. The results achieved in this study are not only helpful for local water resources management, but also for future water utilization planning in the lower reaches region of the Brahmaputra.     

Characteristics of annual runoff variation in major rivers of China

The statistical properties of annual runoff in major rivers of China are studied based on the theory of stochastic process and technology of time series analysis. These properties include the characteristics of intra‐annual and inter‐annual variations of runoff, trends, abrupt changes and periodicities. The new findings from the intensive calculations and appropriate analysis of data in longer period are as follows: (i) compared with the nonuniformity of intra‐annual runoff before 1980, the nonuniformity of intra‐annual runoff in China generally decreased after 1980, except for Huaihe River and Songhua River; (ii) compared with the annual runoff before 1980, the annual runoff in China generally decreased after 1980 except for WangJiaba station in Huaihe River and Ha‐Erbin station in Songhua River; the frequency of continuous low flow and continuous high flow in Haihe River and the downstream of Yellow River is higher than those in other rivers in China; (iii) annual runoff shows a downward trend in major rivers of China especially in Haihe River, Liao River and the midstream and downstream of Yellow River; (iv) there exist certain abrupt changes of annual runoff in major rivers of China; the abrupt change‐points are different among different river basins; and (v) almost periodicities of annual runoff sequences in major rivers of China are generally 20 years below, that is, 3~7 and 12~20 years. The reasons for these changes are mainly caused by climate change and human activities. Copyright © 2011 John Wiley & Sons, Ltd.     

Impact of precipitation and temperature changes on hydrological responses of small-scale catchments in the Ethiopian Highlands

The hydrological response of catchments with different rainfall patterns was assessed to understand the availability of blue and green water and the impacts of changing precipitation and temperature in the Ethiopian Highlands. Monthly discharge of three small-scale catchments was simulated, calibrated, and validated with a dataset of more than 30 years. Different temperature and precipitation scenarios were used to compare the hydrological responses in all three catchments. Results indicate that runoff reacts disproportionately strongly to precipitation and temperature changes: a 24% increase in precipitation led to a 50% increase in average annual runoff, and an average annual rainfall–runoff ratio that was 20% higher. An increase in temperature led to an increase of evapotranspiration and resulted in a decrease in the rainfall–runoff ratio. But a comparison of combined results with different climate change scenarios shows that downstream stakeholders can expect a higher share of available blue water in the future.     

Analyses of trends and causes for variations in runoff and sediment load of the Yellow River

Due to the impacts of globe climate change and human activities, dramatic variations in runoff and sediment load were observed for the Yellow River. Analyses of nearly 65 years' data measured at main hydrologic-stations on the Yellow River from 1950 to 2014 indicated that, except for the Tangnaihai station in the head region, sharp downward trends existed in both the annual runoff and annual sedi-ment load according to the Mann–Kendal trend test;and their abrupt changes occurred in 1986 and in 1980, respectively, according to the rank sum test. Factors affecting the changes in the runoff and sediment load were very complicated. Results indicated that the reducing precipitation and the increasing water consumption were the main causes for the runoff decline, while the impoundment of the Longyangxia Reservoir and its combined operation with the Liujiaxia Reservoir exerted a direct bearing on the abrupt change in the annual runoff. In addition to the sediment load decrease associated with the runoff reduction, the reduced storm intensity, the conducted soil erosion control, and the constructed dam buildings all played an important role in the trends and abrupt changes of sediment load decline.     

Temporal trends of hydro‐climatic variables and runoff response to climatic variability and vegetation changes in the Yiluo River basin,China

The Yiluo River is the largest tributary for the middle and lower reaches of the Yellow River below Sanmenxia Dam. Changes of the hydrological processes in the Yiluo River basin, influenced by the climatic variability and human activities, can directly affect ecological integrity in the lower reach of the Yellow River. Understanding the impact of the climatic variability and human activities on the hydrological processes in the Yiluo River basin is especially important to maintain the ecosystem integrity and sustain the society development in the lower reach of the Yellow River basin. In this study, the temporal trends of annual precipitation, air temperature, reference evapotranspiration (ET₀) and runoff during 1961–2000 in the Yiluo River basin were explored by the Mann‐Kendall method (M‐K method), Yamamoto method and linear fitted model. The impacts of the climatic variability and vegetation changes on the annual runoff were discussed by the empirical model and simple water balance model and their contribution to change of annual runoff have been estimated. Results indicated that (i) significant upwards trend for air temperature and significant downwards trend both for precipitation and ET₀ were detected by the M‐K method at 95% confidence level. And the consistent trends were obtained by the linear fitted model; (ii) the abrupt change started from 1987 detected by the M‐K method and Yamamoto method, and so the annual runoff during 1961–2000 was divided into two periods: baseline period (1961–1986) and changeable period (1987–2000); and (iii) the vegetation changes were the main cause for change of annual runoff from baseline period to changeable period, and climatic variability contributed a little to the change of annual runoff of the Yiluo River. Copyright © 2009 John Wiley & Sons, Ltd.     

Quantitative assessment of the impact of climate variability and human activities on runoff changes: a case study in four catchments of the Haihe River basin,China

Quantitative evaluation of the effect of climate variability and human activities on runoff is of great importance for water resources planning and management in terms of maintaining the ecosystem integrity and sustaining the society development. In this paper, hydro‐climatic data from four catchments (i.e. Luanhe River catchment, Chaohe River catchment, Hutuo River catchment and Zhanghe River catchment) in the Haihe River basin from 1957 to 2000 were used to quantitatively attribute the hydrological response (i.e. runoff) to climate change and human activities separately. To separate the attributes, the temporal trends of annual precipitation, potential evapotranspiration (PET) and runoff during 1957–2000 were first explored by the Mann–Kendall test. Despite that only Hutuo River catchment was dominated by a significant negative trend in annual precipitation, all four catchments presented significant negative trend in annual runoff varying from ?0.859 (Chaohe River) to ?1.996 mm a^?1 (Zhanghe River). Change points in 1977 and 1979 are detected by precipitation–runoff double cumulative curves method and Pettitt's test for Zhanghe River and the other three rivers, respectively, and are adopted to divide data set into two study periods as the pre‐change period and post‐change period. Three methods including hydrological model method, hydrological sensitivity analysis method and climate elasticity method were calibrated with the hydro‐climatic data during the pre‐change period. Then, hydrological runoff response to climate variability and human activities was quantitatively evaluated with the help of the three methods and based on the assumption that climate and human activities are the only drivers for streamflow and are independent of each other. Similar estimates of anthropogenic and climatic effects on runoff for catchments considered can be obtained from the three methods. We found that human activities were the main driving factors for the decline in annual runoff in Luanhe River catchment, Chaohe River catchment and Zhanghe River catchment, accounting for over 50% of runoff reduction. However, climate variability should be responsible for the decrease in annual runoff in the Hutuo River catchment. Copyright © 2012 John Wiley & Sons, Ltd.     

Impact of climate change on water resources in southern Taiwan

This study investigates the impact of climate change on water resources in southern Taiwan. The upstream catchment of Shin-Fa Bridge station in the Kao-Pen Creek basin was the study area chosen herein. The historical trends of meteorological variables, such as mean daily temperature, mean daily precipitation on wet days, monthly wet days, and the transition probabilities of daily precipitation occurrence in each month, at the Kao-Hsiung meteorological station, near the catchments were detected using a non-parametric statistical test. The trends of these meteorological variables were then employed to generate runoff in future climatic conditions using a continuous rainfall–runoff model. The analytical results indicate that the transition probabilities of daily precipitation occurrence significantly influence precipitation generation, and generated runoff for future climatic conditions in southern Taiwan was found to rise during the wet season and decline during the dry season.     

Current runoff variations in the Macta catchment (Algeria): is climate the sole factor?

Abstract

The first objective of this paper is to analyse the trends and change points in the hydroclimatic time series of five representative sub-catchments of the Macta basin, which lies in western Algeria. The second objective is to quantify the role of climate on the trends observed in annual flow time series. This is achieved using hydrological modelling at the multi-annual time step using the Schreiber formulation. The results showed no significant trends on annual rainfall in the 1975–2005 period, a significant increase of temperature and different flow responses to the latter, depending on the catchment considered. Two out of five catchments considered presented a significant flow decrease in the 1975–2005 period with a change point at the beginning of the 1990s. Modelling results suggest that the increase of air temperature is not the sole factor explaining the decrease of annual flow time series in these two catchments.     

Characterisation of hydroclimatological trends and variability in the Lake Naivasha basin,Kenya

Recent hydro‐climatological trends and variability characteristics were investigated for the Lake Naivasha basin with the aim of understanding the changes in water balance components and their evolution over the past 50 years. Using a Bayesian change point analysis and modified Mann–Kendall tests, time series of annual mean, maximum, minimum, and seasonal precipitation and flow, as well as annual mean lake volumes, were analysed for the period 1960–2010 to uncover possible abrupt shifts and gradual trends. Double cumulative curve analysis was used to investigate the changes in hydrological response attributable to either human influence or climatic variability. The results indicate a significant decline in lake volumes at a mean rate of 9.35 × 10⁶ m³ year^?1. Most of the river gauging stations showed no evidence of trends in the annual mean and maximum flows as well as seasonal flows. Annual minimum flows, however, showed abrupt shifts and significant (upward/downward) trends at the main outlet stations. Precipitation in the basin showed no evidence of abrupt shifts, but a few stations showed gradual decline. The observed changes in precipitation could not explain the decline in both minimum flows and lake volumes. The findings show no evidence of any impact of climate change for the Lake Naivasha basin over the past 50 years. This implies that other factors, such as changes in land cover and infrastructure development, have been responsible for the observed changes in streamflow and lake volumes. Copyright © 2015 John Wiley & Sons, Ltd.