ALBERT

Description: Publication date: September 2015 Source: Journal of Hydrology: Regional Studies, Volume 4, Part B Author(s): Alberto Pistocchi, Costanza Calzolari, Francesco Malucelli, Fabrizio Ungaro Study region The plains of Emilia Romagna, Italy. Study focus Urban expansion is among the main causes of increase in flood frequency and intensity in small rural catchments in Europe, and our study region is paradigmatic in this respect. We present here a regional screening-level assessment of soil sealing impacts in terms of increased flood peak discharges and flooding volumes on the secondary drainage network of the plains. We estimate flood peak discharges and flooding volumes through a simple kinematic model with runoff coefficients for the land use of 2008 and 1976. Additionally, we calculate an equivalent compensatory flood detention volume that would enable preserving flood peak discharges as prior to soil sealing (principle of “hydraulic invariance”). The proposed approach is simple and readily applicable to any region facing similar issues, for screening-level assessment of flood hazards over an extended stream network. New hydrological insights for the region The analysis highlights a significant increase in flood hazards throughout the secondary stream network. The impact. Widespread and relatively uniform, is more apparent in smaller catchments and in the case of more permeable soils. This demands retrofitting of the majority of the drainage network and/or significantly higher costs from flooding damages. The analysis suggests that costs of additional flooding after soil sealing may be higher than those of soil sealing impacts compensation through flood detention (hydraulic invariance).

Description: Publication date: September 2015 Source: Journal of Hydrology: Regional Studies, Volume 4, Part B Author(s): M.A. Sarr, O. Seidou, Y. Tramblay, S. El Adlouni Study region The study considers six precipitation stations located in Senegal, West Africa. Senegal is located in the Sahel, an area that is threatened by climate variability and change. Both droughts and extreme rainfall have been an issue in recent years. Study focus Two different statistical downscaling techniques were applied to the outputs of four regional climate models at six selected precipitation stations in Senegal. First, the delta-change method was applied to the mean annual precipitation as well as the 5, 10, 20, 50 and 100-year return period daily precipitation events. Second, a quantile–quantile transformation (QQ) was used to downscale the monthly distributions of precipitation simulated by regional climate models (RCMs). The 5, 10, 20, 50 and 100-year daily precipitation events were afterward calculated. All extreme events were calculated assuming that maximum annual daily precipitations follow the generalized extreme value (GEV) distribution. The two-sided Kolmogorov–Smirnov (KS) test was finally used to assess the performance of the quantile–quantile transformation as well as the GEV distribution fit for the annual maximum daily precipitation. New hydrological insights for the region Results show that the two downscaling techniques generally agree on the direction of the change when applied to the outputs of same RCM, but some cases lead to very different projections of the direction and magnitude of the change. Projected changes indicate a decline in mean precipitation except for one RCM over one region in Senegal. Projected changes in extreme precipitations are not consistent across stations and return periods. The choice of the downscaling technique has more effect on the estimation of extreme daily precipitations of return period equal or greater than ten years than the choice of the climate models.

Description: Publication date: September 2015 Source: Journal of Hydrology: Regional Studies, Volume 4, Part B Author(s): J.S. Lessels, T.F.A. Bishop Study region South eastern Australia. Study focus This region is characterised with rainfall events that are associated with large exports of nutrients and sediments. Many water quality monitoring schemes use a form of event-based sampling to quantify these exports. Previous water quality studies that have evaluated different sampling schemes often rely on continuously monitored water quality data. However, many catchment authorities only have access to limited historical data which consists of event-based and monthly routine samples. Therefore there is a need to develop a method that assesses the importance of sampling events using information from limited historical data. This work presents a simulation based approach using unconditional simulation based on historical stream discharge. Such an approach offers site-specific information on optimal sampling schemes. A linear mixed model is used to model the relationship between total phosphorus and stream discharge and the auto-correlation of total phosphorus. New hydrological insights for the region The inclusion of event-based sampling improved annual load estimates of all sites with a maximum RMSE difference of 16.11 tonnes between event-based and routine sampling. Based on the accuracy of annual loads, event-based sampling was found to be more important in catchments with a large relief and high annual rainfall in this region. Using this approach, different sampling schemes can be compared based on limited historical data.

Description: Publication date: September 2015 Source: Journal of Hydrology: Regional Studies, Volume 4, Part B Author(s): Hayet Chihi, Ghislain de Marsily, Habib Belayouni, Houcine Yahyaoui Study region “Jeffara de Medenine” aquifer system in south-eastern Tunisia. Study focus This study investigates the role of fault structures in the distribution of hydrogeochemical facies and groundwater compartmentalization for the aquifer system. New hydrological insights for the region The proposed methodology, including seismic structural study, hierarchical cluster analysis and geostatistical methods, allowed an efficient multi-element characterization of the spatial patterns of the structural elements in the aquifers and of the hydrogeological parameters used in a spatial cross-correlation to explore the dependence of the geochemical properties in each “geochemical population” on the hosting structural compartment to delineate the different geochemical compartments. The tectonic studies showed that the lateral extent of the aquifers is controlled by normal faults. The multivariate statistical analysis revealed a strong spatial coherence between hydrogeochemical facies clustering and the reservoir compartments at both large and small scales. The kriged maps of major-ion concentrations and of total dissolved solids in the aquifers were then analyzed and compared with the reservoir facies distribution for each compartment, the geometric characteristics of the aquifer, and the piezometric level trends. This allowed to characterize the hydraulic behavior of the Medenine fault and to understand the underlying physical and chemical processes having led to the spatial distribution of the geochemical properties, and thus, the hydrogeochemical functioning of the aquifers.

Description: Publication date: September 2015 Source: Journal of Hydrology: Regional Studies, Volume 4, Part B Author(s): Marnie L. Atkins, Isaac R. Santos, Damien T. Maher Study region This study investigates dissolved methane distribution in groundwater from the Richmond River Catchment (New South Wales, Australia) before proposed coal seam gas (CSG, or coal bed methane) development. Study focus Unconventional gas exploration has rapidly expanded in recent years. However, the impact of these operations on groundwater systems is poorly understood. A total of 91 groundwater samples were analyzed from 6 geological units. Our observations act as regional baseline research prior to CSG extraction and may assist with long term impact assessment. New hydrological insights for the region Methane was found in all geological units ranging between 0.26 and 4427 μg L −1 (median 10.68 μg L −1 ). Median methane concentrations were highest in chloride-type groundwater (13.26 μg L −1 , n = 58) while bicarbonate-type groundwater had lower concentrations (3.71 μg L −1 ). Groundwater from alluvial sediments had significantly higher median methane concentrations (91.46 μg L −1 ) than groundwater from both the basalt aquifers (0.7 μg L −1 ) and bedrock aquifers (4.63 μg L −1 ); indicating geology was a major driver of methane distribution. Methane carbon stable isotope ratios ranged from –90.9‰ to –29.5‰, suggesting a biogenic origin with some methane oxidation. No significant correlations were observed between methane concentrations and redox indicators (nitrate, manganese, iron and sulphate) except between iron and methane in the Lismore Basalt ( r 2 = 0.66, p 〈 0.001), implying redox conditions were not the main predictor of methane distribution.

Description: Publication date: September 2015 Source: Journal of Hydrology: Regional Studies, Volume 4, Part B Author(s): Z. Zomlot, B. Verbeiren, M. Huysmans, O. Batelaan Study focus Groundwater is of strategic importance. The accurate estimation of groundwater recharge and assessing the fundamental controlling factors are therefore of utmost importance to protect groundwater systems. We used the spatially-distributed water-balance model WetSpass to estimate long-term average recharge in Flanders. We validated recharge rates with base flow estimates of 67 daily stream flow records using the hydrograph analyses. To this end we performed principal component analysis, multiple linear regression analysis and relative importance analysis to assess the controlling factors of the spatial variation of recharge and base flow with the influencing watershed characteristics. New hydrological insights for the region The average resulting recharge is 235 mm/year and occurs mainly in winter. The overall moderate correlation between base flow estimates and modeled recharge rates indicates that base flow is a reasonable proxy of recharge. Groundwater recharge variation was explained in order of importance by precipitation, soil texture and vegetation cover; while base flow variation was strongly controlled by vegetation cover and groundwater depth. The results of this study highlight the important role of spatial variables in estimation of recharge and base flow. In addition, the prominent role of vegetation makes clear the potential importance of land-use changes on recharge and hence the need to include a proper strategy for land-use change in sustainable management of groundwater resources.

Description: Publication date: September 2015 Source: Journal of Hydrology: Regional Studies, Volume 4, Part B Author(s): B. Ibrahim, D. Wisser, B. Barry, T. Fowe, A. Aduna Study region Hydrological observation networks in the West African region are not dense and reliable. Furthermore, the few available discharge data often present significant gaps. The Volta basin, the second largest transboundary basin in the region, is a typical example of a basin with inadequate hydrological networks. Study focus In this study, a prediction approach to determine monthly discharge in ungauged watersheds is developed. The approach is based on the calibration of two conceptual models for gauged watersheds and an estimation of models’ parameters from the physical and climatic characteristics of the watersheds. The models’ parameters were determined for each ungauged watershed through two different methods: the multiple linear regressions and the kriging method. The two methods were first validated on five gauged watersheds and then applied to the three ungauged watersheds. New hydrological insights for the region The application of the two hydrological models on the eight watersheds helped to produce relevant monthly runoff and to establish the annual hydrological balances from 1970 to 2000 for both gauged and ungauged watersheds. The developed method in this study could therefore help estimate runoff time series, which are of crucial importance when it comes to design hydraulic structures such as small reservoirs.

Description: Publication date: Available online 14 August 2015 Source: Journal of Hydrology: Regional Studies Author(s): Mayank Shekhar, Amalava Bhattacharyya Study region Zemu Chuu (river), Lachen, North Sikkim, Eastern Himalaya India. Study focus Using tree-ring data of fir ( Abies densa ) the temporal variation of 222 years January–April mean discharge of Zemu Chuu, upper reaches of the Teesta River at Lachen, North Sikkim Eastern Himalaya was investigated. This was based on linear regression reconstruction model which explained variance of 50.1% during calibration period (AD 1976–1996). The model was verified by reduction of error (RE), sign test (ST), product mean test (Pmt), root mean square error (RMSE) and Durbin–Watson test (DW). The RE never falls below zero suggesting the model had explanatory power over the entire period of reconstruction. New hydrological insights for the region The explored strong relationship between tree ring records and instrumental data enable to develop mean January–April months (premonsoon) river discharge of Zemu Chuu from remote area of Sikkim. Reconstructed data reveals high stream-flow when it is more than the mean plus one standard deviation and as low when flow is less than the mean minus one standard deviation. There were such 23 high discharge and 21 extremely low years over the past AD 1775–1996. This premonsoon reconstruction of river flow would be of great significance when scarcity of water is acute in the North East Himalaya.