Denis Ruelland

Hydrology under change: an evaluation protocol to investigate how hydrological models deal with changing catchments

Abstract Testing hydrological models under changing conditions is essential to evaluate their ability to cope with changing catchments and their suitability for impact studies. With this perspective in mind, a workshop dedicated to this issue was held at the 2013 General Assembly of the International Association of Hydrological Sciences (IAHS) in Göteborg, Sweden, in July 2013, during which the results of a common testing experiment were presented. Prior to the workshop, the participants had been invited to test their own models on a common set of basins showing varying conditions specifically set up for the workshop. All these basins experienced changes, either in physical characteristics (e.g. changes in land cover) or climate conditions (e.g. gradual temperature increase). This article presents the motivations and organization of this experiment—that is—the testing (calibration and evaluation) protocol and the common framework of statistical procedures and graphical tools used to assess the model performances. The basins datasets are also briefly introduced (a detailed description is provided in the associated Supplementary material).

Long-term monitoring of land cover changes based on Landsat imagery to improve hydrological modelling in West Africa

The spatial and temporal variability of land cover changes is a fundamental parameter to integrate when modelling water resources in order to reproduce the relations between rainfall and surface flow more precisely. This is particularly important in West Africa, where the land cover has been changing for more than 40 years under the combined impact of climatic effects and human activities. In this study, we evaluated the potential of Landsat imagery to monitor the vegetation cover in the upper Niger watershed (120 000 km2) using archive images from MSS, TM and ETM+ sensors covering three periods of time around 1975, 1985, and 2000. Because of the heterogeneity of the acquisition dates, the spatial and spectral resolution of the images, and the scale of analysis, we chose a simple system of classification. Pretreatments were applied to reduce variations between the images. Vegetation indices (NDVI) were then calculated and subsequently thresholded using the same land‐cover classification system. The thresholds were then optimized by automated recursive calculations of confusion matrices and control parcels. Our results revealed that although the accuracy was not perfect, it was nevertheless possible to estimate changes using an unconventional spatio‐temporal scale. The resulting changes were characterized by a moderate trend to deforestation with a corresponding increase in bare soils, soils with sparse vegetation, and shrublands. The spatial layers produced were then combined with a soil map to incorporate changes in surface conditions in the hydrological modelling of the Niger River.

Current state of Mediterranean water resources and future trends under climatic and anthropogenic changes

Abstract The Mediterranean basin has been identified as one of the worlds regions most vulnerable to climatic and anthropogenic changes. A methodology accounting for the basin specific conditions is developed to assess the impacts of these changes on water resources. Based on global climate projections and water-use scenarios inspired by national reports, the current water stress state is addressed first and then it is explored for the medium-term. Currently, the southern and eastern rims are experiencing high to severe water stress. By the 2050 horizon, this stress could increase over the whole Mediterranean basin, notably because of a 30–50% decline in freshwater resources as a result of climate change. In addition, under a business-as-usual water-use scenario, total water withdrawals are projected to double on the southern and eastern rims. These worrying trends indicate the need to develop mitigation scenarios. In accord with the Mediterranean Strategy for Sustainable Development, an alternative water-use scenario based on improvements in the efficiency of water distribution networks and of irrigated agriculture is investigated. Such progress would stabilize total water withdrawals over the Mediterranean basin and even make them decrease (10–40%) in many northern catchments. Water stress could thus be tempered in some eastern catchments and remain low on the northern rim. This study highlights the importance of developing sustainable development strategies to cope with climatic and anthropogenic changes in order to explore their impacts at regional scales. It supports the need to focus on the most vulnerable areas within the Mediterranean basin. Editor Z.W. Kundzewicz Citation Milano, M., Ruelland, D., Fernandez, S., Dezetter, A., Fabre, J., Servat, E., Fritsch, J.-M., Ardoin-Bardin, S., and Thivet, G., 2013. Current state of Mediterranean water resources and future trends under climatic and anthropogenic changes. Hydrological Sciences Journal, 58 (3), 498–518.

Assessing the long-term impact of climatic variability and human activities on the water resources of a meso-scale Mediterranean catchment

Abstract This article addresses the critical need for a better quantitative understanding of how water resources from the Hérault River catchment in France have been influenced by climate variability and the increasing pressure of human activity over the last 50 years. A method is proposed for assessing the relative impacts of climate and growing water demand on the decrease in discharge observed at various gauging stations in the periods 1961–1980 and 1981–2010. An annual water balance at the basin scale was calculated first, taking into account precipitation, actual evapotranspiration, water withdrawals and water discharge. Next, the evolution of the seasonal variability in hydroclimatic conditions and water withdrawals was studied. The catchment was then divided into zones according to the main geographical characteristics to investigate the heterogeneity of the climatic and human dynamics. This delimitation took into account the distribution of climate, topography, lithology, land cover and water uses, as well as the availability of discharge series. At the area scale, annual water balances were calculated to understand the internal changes that occurred in the catchment between both past periods. The decrease in runoff can be explained by the decrease in winter precipitation in the upstream areas and by the increase during summer in both water withdrawals and evapotranspiration in the downstream areas, mainly due to the increase in temperature. Thus, water stress increased in summer by 35%. This work is the first step of a larger research project to assess possible future changes in the capacity to satisfy water demand in the Hérault River catchment, using a model that combines hydrological processes and water demand. Editor Z.W. Kundzewicz Citation Collet, L., Ruelland, D., Borrell-Estupina, V., and Servat, E., 2014. Assessing the long-term impact of climatic variability and human activities on the water resources of a meso-scale Mediterranean catchment. Hydrological Sciences Journal, 59 (8), 1457–1469. http://dx.doi.org/10.1080/02626667.2013.842073

Nitrate pollution risk assessment : from the model to the indicator

The diffuse nature of nitrate pollution makes it difficult to evaluate existing or planned measures to reduce it. Tools have therefore been developed to assess this pollution, ranging from simple indicators to complex models. The aim of this paper is to compare indicators and models by analysing results obtained from their individual application to the same area. The pros and cons of each approach are evaluated in terms of both the conditions of their implementation and the results obtained. This comparison helps to guide the choice of a methodology. Rules governing that choice are set in relation to the nature of the diagnosis to perform and the characteristics of the area under study.

Climate change impacts on surface water resources in the Rheraya catchment (High Atlas, Morocco)

ABSTRACT This study aimed to quantify possible climate change impacts on runoff for the Rheraya catchment (225 km2) located in the High Atlas Mountains of Morocco, south of Marrakech city. Two monthly water balance models, including a snow module, were considered to reproduce the monthly surface runoff for the period 1989‒2009. Additionally, an ensemble of five regional climate models from the Med-CORDEX initiative was considered to evaluate future changes in precipitation and temperature, according to the two emissions scenarios RCP4.5 and RCP8.5. The future projections for the period 2049‒2065 under the two scenarios indicate higher temperatures (+1.4°C to +2.6°C) and a decrease in total precipitation (−22% to −31%). The hydrological projections under these climate scenarios indicate a significant decrease in surface runoff (−19% to −63%, depending on the scenario and hydrological model) mainly caused by a significant decline in snow amounts, related to reduced precipitation and increased temperature. Changes in potential evapotranspiration were not considered here, since its estimation over long periods remains a challenge in such data-sparse mountainous catchments. Further work is required to compare the results obtained with different downscaling methods and different hydrological model structures, to better reproduce the hydro-climatic behaviour of the catchment. EDITOR M.C. Acreman ASSOCIATE EDITOR R. Hirsch

Analysis of 1982–2006 Sudano-Sahelian vegetation dynamics using NOAA-AVHRR NDVI data and normalized rain-use efficiency

Land cover dynamic has to be taken into account to analyze changes in water resources, especially in vulnerable environment such as the Bani catchment in Mali. To study the land cover changes, we used NDVI AVHRR time series (1982-2006, 8 km spatial resolution), and monthly rainfall data from 65 stations. To interpret the NDVI trends in terms of land cover changes, we had to eliminate the inter-annual rainfall variability. We used the concept of the Rain Use Efficiency (RUE) which is the ratio between NDVI (a proxy of the Net Primary Production) and precipitation. RUE and rainfall were calculated and modeled on a 0.5° × 0.5° grid scale. For each cell we normalized the evolution of the RUE through time (RUE_cor), and calculated its trend over the 25 years period. The results indicate that RUE_cor is stable or in light increase for most of the grid cells. In areas where water is not a limiting factor of NPP, this trend is positively correlated to the fraction of cropped area changes, as determined from a couple of Landsat images acquired during a similar period. However, RUE is a complex concept and further investigations are needed to consolidate our results and conclusions.

Spatiotemporal characterization of current and future droughts in the High Atlas basins (Morocco)

Over the past decades, drought has become a major concern in Morocco due to the importance of agriculture in the economy of the country. In the present work, the standardized precipitation index (SPI) is used to monitor the evolution, frequency, and severity of droughts in the High Atlas basins (N’Fis, Ourika, Rhéraya, Zat, and R’dat), located south of Marrakech city. The spatiotemporal characterization of drought in these basins is performed by computing the SPI with precipitation spatially interpolated over the catchments. The Haouz plain, located downstream of these basins, is strongly dependent on water provided by the mountain ranges, as shown by the positive correlations between the normalized difference vegetation index (NDVI) in the plain and the 3, 6, and 12-month SPI in the High Atlas catchments. On the opposite, no significant correlations are found with piezometric levels of the Haouz groundwater due to intensified pumping for irrigation in the recent decades. A relative SPI index was computed to evaluate the climate change impacts on drought occurrence, based on the projected precipitation (2006–2100) from five high-resolution CORDEX regional climate simulations, under two emission scenarios (RCP 4.5 and RCP 8.5). These models show a decrease in precipitation towards the future up to − 65% compared to the historical period. In terms of drought events, the future projections indicate a strong increase in the frequency of SPI events below − 2, considered as severe drought condition.

Consistency of satellite precipitation estimates in space and over timecompared with gauge observations and snow-hydrological modellingin the Lake Titicaca region

This paper proposes a protocol to assess the space-time consistency of satellite precipitation estimates (SPEs) 10 according to various indicators including: (i) direct comparison of SPEs with 72 precipitation gauges; (ii) sensitivity of streamflow modelling to SPEs at the outlet of four basins; and (iii) the sensitivity of distributed snow models to SPEs using a MODIS snow product as reference in an unmonitored mountainous area. The protocol was applied successively to four different time windows (2000‒2004, 2004‒2008, 2008‒2012 and 2000‒2012) to account for the space-time variability of the SPEs and to a large dataset composed of 12 SPEs (CMORPH-RAW, CMORPH-CRT, CMORPH-BLD, CHIRP, CHIRPS, 15 GSMaP, MSWEP, PERSIANN, PERSIANN-CDR, TMPA-RT, TMPA-Adj and SM2Rain), an unprecedented comparison. The aim of using different space-time scales and indicators was to evaluate whether the efficiency of SPEs varies with the method of assessment, time window and location. Results revealed very high discrepancies between SPEs compared to precipitation gauge observations. Some SPEs (CMORPH‒RAW, CMORPH‒CRT, GSMaP, PERSIANN, TMPA‒RT and SM2Rain) are unable to estimate regional precipitation whereas the others (CHIRP, CHIRPS, CMORPH‒BLD, MSWEP, 20 PERSIANN‒CDR and TMPA‒Adj) produce a realistic representation despite recurrent spatial limitation over regions with contrasted emissivity, temperature and orography. In nine out of ten of the cases studied, streamflow was more realistically simulated by the hydrological model tested when SPEs were used as forcing precipitation data rather than precipitation derived from the available precipitation gauge networks. Interestingly, the potential of SPEs to reproduce the observed streamflow varied significantly depending on the basin and period considered and did not systematically corroborate SPE 25 potential compared with gauge precipitation observations. SPE’s ability to reproduce the duration of MODIS-based snow cover also showed variable consistency over time with poorer simulations in comparison to those simulated from available precipitation gauges. Using snow cover simulations as indicator led to a different efficiency ranking of the SPEs that the ones obtained when using observed gauge precipitation and streamflow. SPEs thus present space-time errors that may not be detected when short time windows and/or scarce gauge networks and/or single indicators are used, underlining how 30 important it is to carefully consider their space-time consistency before using them for hydro-climatic studies. Moreover SPE Hydrol. Earth Syst. Sci. Discuss., https://doi.org/10.5194/hess-2018-316 Manuscript under review for journal Hydrol. Earth Syst. Sci. Discussion started: 13 September 2018 c