A document by Ali Aldrees. Click on the document to view its contents.

The rapid expansion of impermeable surfaces in cities has a major impact on urban hydrology. Infiltration of rainwater is reduced and water runs off faster with higher runoff peaks. Urban trees as stormwater management tools are becoming more relevant to reduce flood risks in addition to other ecosystem services. An in-situ field experiment to measure throughfall on Norway maple ( Acer platanoides) and small-leaved lime ( Tilia cordata) was conducted to determine the interception of solitary urban trees with different degrees of surface sealing in the city of Freiburg, Germany. The relationships between rainfall characteristics, tree morphological traits, and the interception behavior were investigated with eight trees per species. 76 recorded rainfall events were evaluated from April to September 2021. Average interception values were higher for small-leaved lime (70.3 ± 6.6%) than for Norway maple (54.8 ± 10.3%) and hence much higher than in a typical forested environment. The average interception loss of all recorded events was 2.58 ± 0.60 mm for Norway maple and 3.73 ± 0.29 mm for small-leaved lime. For both tree species, significant linear correlations were found between the relative interception and other factors like rainfall depths, the leaf area index (LAI), and the plant area index (PAI) (adj.R 2 > 0.45). In contrast to Norway maple, small-leaved lime also showed significant relationships of several tree morphological parameters with the interception (adj.R 2 > 0.43). LAI, which also effects the interception, of both tree species significantly decreased with the degree of surface sealing. Our results provide a better understanding of the interception process of solitary trees for different urban sites and allows to parameterize interception based on measurable properties. However, further field experiments with various tree species need to be conducted to obtain a larger database for typical parameters in models and to support urban planners in managing stormwater runoff.

Irrigation activities are a major control on water movement and storage in irrigated river valleys in the Intermountain West, USA. Particularly in dry years, surface water diversions can deplete streams over the summer irrigation season, leading to more variable stream temperatures and increased risk for resident aquatic species. Cooler lateral inflows derived from irrigation activities can mitigate the impacts of depletion by buffering main channel stream temperatures. Given the increasing susceptibility of depleted streams to climate and land use changes, understanding stream temperature patterns and controls in these systems is critical. We used intensive field monitoring over three summers and thermal aerial imagery to characterize stream temperature patterns and irrigation influences in a 2.5 km reach of a small agricultural stream in northern Utah. Considering variable hydrology, weather, channel morphology, diversions, and lateral inflows we found stream temperatures to be relatively insensitive to flow depletion or lateral inflows in a wet year but very sensitive in drier years. Irrigation-related lateral inflows reduced longitudinal warming and diel variability during drier years and at times prevented temperatures from reaching stressful or lethal limits. Reaches with substantial lateral inflow contributions also had a greater areal proportion of low temperatures and spatial temperature diversity. These trends were enhanced by differences in channel morphology, with greater spatial and temporal variability in multi-thread than single-thread reaches. Study results highlight critical flow and weather conditions driving increased temperature variability that will likely become more extreme with additional climate change related reductions in baseflow. Regardless of the cause, this study highlights that decreased instream flows increase the importance of identifying, quantifying, and maintaining lateral inflows to maintain instream temperatures and preservation of these inflows should be considered in future water management decisions.

The projected climate change for Norway through the 21st century predicts that the temperature will increase significantly. Events with heavy rainfall will be more intense and occur more frequently. Rain floods will increase in magnitude and also occur more frequently. Extreme flooding and heavy rain will significantly impact the sediment dynamics in rivers. In the mountain areas, floods are often associated with erosion, transport and deposition of coarse sediment along the streams. These processes are related to bed load transport and pose a hazard in addition to the elevated water discharge and have to be included in management plans for river basins. This paper studies the bed load delivery from sources that contribute the most to the sediment budget in the Gudbrandsdalslågen river basin during the large magnitude floods in 2011 and 2013. More than 100 debris slides and debris flow were triggered in the tributary river Veikleåi by the heavy rain and snowmelt during these floods.  The volume of the contribution from debris flows and erosion and deposition of the river bed was determined by subtracting digital elevation models acquired during repeated airborne LIDAR surveys. In the river Dørja the supply of sediment from a number of debris flows caused extensive aggradation and channel changes. In their new position, lateral erosion by these channels triggered slides on the adjacent slopes. The contributing volumes of debris flows, lateral erosion, and river-bed erosion and deposition were determined from the LIDAR surveys. Relations obtained from studies of sediment transport in modern glacier rivers were used to obtain estimates of the ratio of bed load versus suspended load derived from the Pleistocene moraine deposits. Several monitoring stations using conventional methods for measuring bed load and suspended load recorded very large volumes of sediment delivery during both of the extreme floods. The results of the study may be used to identify necessary locations for check-dams and erosion protection works in order to adapt to climate change.

A regional coupled approach to water cycle prediction is demonstrated for the 4-month period from November 2013 to February 2014 through analysis of precipitation, soil moisture, river flow and coastal ocean simulations produced by a km-scale atmosphere-land-ocean coupled system focussed on the United Kingdom (UK), running with horizontal grid spacing of around 1.5 km across all components. The Unified Model atmosphere component, in which convection is explicitly simulated, reproduces the observed UK rainfall accumulation (r2 of 0.62 for daily accumulation), but there is a notable bias in its distribution – too dry over western upland areas and too wet further east. The JULES land surface model soil moisture state is shown to be in broad agreement with a limited number of cosmic-ray neutron probe observations. A comparison of observed and simulated river flow shows the coupled system is useful for predicting broad scale features, such as distinguishing high and low flow regions and times during the period of interest but are shown to be less accurate than optimised hydrological models. The impact of simulated river discharge on NEMO model simulations of coastal ocean state is explored in the coupled system, with comparisons provided relative to experiments using climatological river input and no river input around the UK coasts. Results show that the freshwater flux around the UK contributes of order 0.2 psu to the mean surface salinity, and comparisons to profile observations give evidence of an improved vertical structure when applying simulated flows. This study represents a baseline assessment of the coupled system performance, with priorities for future model developments discussed.

Groundwater-surface water interactions (GSI) connect rivers and streams with riparian areas and the adjacent aquifer. Although these interactions represent a substantial control of quantity and quality of both groundwater and surface water, knowledge on GSI along rivers at the regional scale, particularly inland waterways, is still limited. We investigated GSI along the river Moselle, an important federal inland waterway in Germany, by using radon and tritium to identify gaining and losing stream conditions, respectively. Gaining stream conditions were identified by continuously measuring radon along the river as part of boat surveys using a high spatial resolution (every two km) during intermediate (October 2020) and near low flow conditions (August/September 2021). Quarterly tritium inventories from 2017 to 2021 revealed losses of up to 27 % due to losing stream conditions at the upstream locations of damns (particularly near the hydroelectric power plant Lehmen) while gains (up to 51 %) likely triggered by a flood-induced mass transfer of water from the aquifer back into the river. Using radon mass balance modeling, good agreements of simulated versus measured radon data with respect to two groundwater end-member scenarios were obtained during intermediate flow (Spearman’s ρ: 0.97 and 0.99; MAE: 10.1 and 3.4 Bq l -1) and near low flow (Spearman’s ρ: 0.97 and 0.99; MAE: 11 and 6.5 Bq l -1). Important groundwater inflow was limited to the meander of Detzem, where cumulated groundwater inflow of about 19 m 3 s -1 (9.5 % of total discharge) and 4.2 m 3s -1 (3.8 % of total discharge) was simulated during intermediate and near low flow, respectively. However, these groundwater contributions were relatively low compared to alpine streams, for example. Finally, the study will help to better identify and quantify GSI at the regional scale and provide methodological guidance for future studies focusing on inland waterways.

Evapotranspiration (ET) plays an important role in integrated water resource planning, development and management. This process is particularly relevant in semiarid regions. The aim of the present study is to compare the actual spatial and temporal evapotranspiration (ETa) patterns and temporal trends in two semiarid forests, one in Brazil (Aiuaba) and the other in Spain (Valladolid). We used the Surface Energy Balance Algorithm for Land (SEBAL) to assess the effect of climatic variation in both areas. In the Brazilian semiarid forest, Caatinga is the main vegetation, while it is stone pine in Spain. For this purpose, 69 Landsat-5 and 42 Landsat-8 images (1995 – 2019) were used. The Mann-Kendall test was applied to assess the occurrence of trends in precipitation, temperature and potential evapotranspiration data; and the Temporal Stability Index (TSI) to know which areas have greater seasonal ETa. The annual amplitude of the potential evapotranspiration (ET0) is the same in both areas, however, the Caatinga values are higher. In the Caatinga forest, when ET0 presents its highest values throughout the year, ETa presents the lowest, and vice versa. In the Pinares forest, ETa follows the ET0 dynamics during the year, and the difference between ET0 and ETa is maximum during the summer. The Caatinga forest showed a greater spatial variation of ETa than the Pinares forest as well as a greater extension with lower temporal stability of ETa than the Pinares forest. Both the Caatinga forest and the Pinares forest showed significant annual trends of increase for ET0 and ETa: 3.5 mm and 2.2 mm, and 7.0 mm and 3.9 mm, respectively.

The streambed is the critical interface between the aquatic and terrestrial systems and hosts important biogeochemical hot spots within river corridors. Although the streambed characteristics are significantly different from those of its surrounding soil, the streambed itself has not been explicitly represented in watershed models. We developed an integrated hydrologic model that explicitly incorporated a streambed layer to examine the hydrological effects of streambed characteristics including hydraulic conductivity (K), layer thickness, and width on the exchange fluxes across the streambed as well as the streamflow at the watershed outlet. The numerical experiments were performed in the American River Watershed, a headwater, mountainous watershed within the Yakima River Basin in central Washington. Despite having a negligible effect on the watershed streamflow, an explicit representation of the streambed with distinctive properties dramatically changed the magnitude and variability of the exchange flux. In general, larger streambed K along with a thicker streambed layer induced larger exchange fluxes. The exchange flux was most sensitive to the streambed width or the mesh resolution of the streambed. A smaller streambed width (or a finer streambed resolution) increases exchange fluxes per unit area while reducing the overall exchange volumes across the entire streambed. The amount of baseflow decreased by 6% as the streambed width decreased from 250 m to 50 m. This finding is important because these hydrological changes may in turn affect the exchange of nutrients and contaminants between surface water and groundwater and the associated biogeochemical processes. Our work demonstrated the importance of representing streambed in fully distributed, process-based watershed models in better capturing the exchange flow dynamics in river corridors.

Fully coupled atmospheric-hydrological models allow a more realistic representation of the land surface–boundary layer continuum, representing both high-resolution land-surface/subsurface water lateral redistribution and the related feedback towards the atmosphere. This study evaluates the potential contribution of the fully coupled approach in extended-range mesoscale hydrometeorological ensemble forecasts. Previous studies have shown, for deterministic simulations, that the effect of fully coupling for short-range forecasts is minor compared to other sources of uncertainty, however, it becomes not negligible when increasing the forecast period. Through a proof-of-concept consisting of an ensemble (50 members from the ECMWF Ensemble Prediction System) seven-days-in-advance forecast of a high impact event affecting the Calabrian peninsula (southern Italy, Mediterranean basin) on November 2019, the paper elucidates the extent to which the improved representation of the terrestrial water lateral transport in the Weather Research and Forecasting (WRF) – Hydro modeling system affects the ensemble water balance, focusing on the precipitation and the hydrological response, in terms of both soil moisture dynamics and streamflow in 14 catchments spanning over 42% of the region. The fully coupled approach caused an increase of surface soil moisture and latent heat flux from land in the days preceding the event, partially affecting the lower Planetary Boundary Layer. However, when shoreward moisture transport from surrounding sea rapidly increased becoming the dominant process, only a weak signature of soil moisture contribution could be detected, resulting in only slightly higher precipitation forecast and not clear variation trend of peak flow, even though the latter variable increased up to 10% in some catchments. Overall, this study highlighted a remarkable performance of the medium-range ensemble forecasts, suggesting a profitable use of the fully coupled approach for forecasting purposes in circumstances in which soil moisture dynamics is more relevant and needs to be better addressed.

Seasonal snowmelt pulses are the dominant hydrologic feature of most alpine catchments. The majority of annual export of water, carbon and nitrogen occurs within a short window of only a few weeks. This observation has largely been based on relatively infrequent manual sampling, and our understanding of responses to finer-scale variation, “pulses within the pulse”, is critically incomplete. Here, we combine high-frequency sensor measurements of dissolved organic carbon (DOC) and nitrate (NO3-N) with historical grab sample data from a high altitude stream in the Rocky Mountains of Colorado. We characterize the linkages between precipitation, snowpack, streamflow, and solute export, over time scales ranging from decades to minutes. At all time scales, discharge (Q) variation was several orders of magnitude larger than concentration (C) variation, making it the dominant control on solute flux rates. Interannual variation in Q, and by extension solute export, appeared correlated to the depth of the winter snowpack, and how late into the spring the snowpack persisted. Seasonally, we observed clockwise C-Q hysteresis, with solute stores becoming depleted as the melt pulse proceeds. Using the sensor data however, we were able to observe individual events. In contrast to the seasonal patterns, these events enriched concentrations, suggesting the persistence of additional DOC and NO3-N stores which can be mobilized within, and even after the main seasonal snowmelt pulse. The historical data suggest that reduced snowpack and earlier snowmelt in the coming decades may result in reduced export of DOC and NO3-N. The sensor data however make this conclusion uncertain, as rain on snow events, which are expected to become more prevalent, appear equally capable of mobilizing solutes.

Snowmelt drives a large portion of streamflow in many mountain areas of the world. However, the water pathways since snow melts until water reaches the streams, and its associated transit time is still largely unknown. Such processes are important for drawing conclusions about the hydrological role of the upstream snowpack after melting. This work analyzes for first time the influence of snowmelt on spring streamflow in years of different snow accumulation and duration, in an alpine catchment of the central Spanish Pyrenees. A multi-approach research was performed, by combining the analysis of climatic, snow, streamflow, piezometric levels, water temperature, electrical conductivity and isotopic (δ 18O) data. Results show that snow played a preeminent role on the hydrological response of the catchment during spring. Liquid precipitation during the melting period also determined the shape of the spring hydrographs. When snow cover disappeared from the catchment, soil water storage and streamflow showed a sharp decline. Consequently, streamflow electrical conductivity, temperature and δ 18O showed a marked tipping point towards higher values. The fast hydrological response of the catchment to snow and meteorological fluctuations, as well as the marked diel fluctuations of streamflow δ 18O during the melting period, strongly suggests soil storage was small, leading to short meltwater transit times. As a consequence of this hydrological behavior, independently of the amount of snow accumulated and of melting date, summer streamflow remained always low, with small runoff peaks driven by rainfall events. The expected reduction of snow accumulation and duration in the area in a next future will bring an earlier snowmelt and rise of stream water temperature. However, given the low storage capacity of the catchment and the contribution of rainfall events to spring runoff, the annual water balance and the runoff seasonality of the catchment would not change drastically.

Post-fire managers throughout the world use predictive models to estimate potential erosion risks to aid in evaluating downstream impacts of increased runoff and erosion, and to target critical areas within a fire for applying mitigation practices. Erosion prediction can be complicated by forest road networks. Using GIS technology and a soil erosion model, this study evaluated the effect of roads on erosion and sediment yield following a wildfire, and whether the predictive models were providing reasonable results. The GeoWEPP model was used to simulate onsite erosion and offsite sediment delivery before and after fire disturbance. A 2-m resolution DEM was used as the terrain layer. Erosion rates in excess of 4 Mg ha-1 yr-1 were predicted mainly from the moderate and high severity burn areas. Roads influenced both flow path and sub-catchment delineations, affecting the spatial distribution of sediment detachment and transport. Through that influence, roads tended to reduce estimated erosion on slopes below the roads, but road fillslopes and steep channels were areas of significant increases in erosion risks. Measured deposition amounts along roads and in sediment basins were similar to predicted amounts. The results confirm that road prisms, culverts and road ditches greatly influence sedimentation processes after wildfire, and they present opportunities to detain eroded sediments before they reach downstream water bodies.

Runoff and erosion can increase after wildfires, but little is known about the effects of wildfire plus post-fire salvage logging, or mitigating these effects. Past research has identified soil compaction and reduced surface cover as controls on runoff and erosion, but the relative contributions of these changes are not clear. Two years after high severity burning by the 2015 Valley Fire in California, replicated rainfall simulations were carried out in four soil conditions across compaction and cover factors: uncompacted/compacted by logging machinery and bare soil/60% wood slash-cover. Runoff after 71 mm of rainfall totaled 27 mm in the uncompacted bare plots and 39 mm in the compacted bare plots. Runoff in the slash-covered plots decreased by 50% and 33% as compared to the uncompacted and compacted bare plots, respectively, although none of the differences in runoff were significant. Rainsplash averaged 30 g for the bare plots, regardless of compaction, and decreased significantly by 70% on slash-covered plots. Sediment yield totaled 460 and 818 g m-2 for the uncompacted and compacted bare plots, respectively, and slash significantly reduced these amounts by 72% and 69%, respectively. Our results showed that post-fire soil erosion in high severity burned unlogged areas was still very high two years after the wildfire. The combination of wildfire and salvage logging doubled soil erosion by increases in both runoff amount and sediment concentration. Antecedent soil moisture (dry or wet) was the dominant factor for runoff, while surface cover was the dominant factor for erosion and sediment delivery. Covering the soil with slash reduced both runoff and erosion, suggesting this treatment would reduce long‐term sediment delivery from burned areas and skid trails. Saturated hydraulic conductivity (Ks) and interrill erodibility (Ki) calculated from these simulations confirmed previous research and will support modeling efforts related to wildfire and post-fire salvage logging.

It is widely recognized that multi-year drought can induce changes in catchment hydrological behaviors. However, at present, our understanding about multi-year, drought-induced changes in catchment hydrological behaviors and its driving factors at the process level is still very limited. This study proposed a new approach using a data assimilation technique with a process-based hydrological model to detect multi-year drought-induced changes in catchment hydrological behaviors and to identify driving factors for the changes in an unimpaired Australian catchment (Wee Jasper) which experienced prolonged drought from 1997 to 2009. Modelling experiments demonstrated that the multi-year drought caused a significant change in the catchment rainfall-runoff relationship, indicated by significant step changes in the estimated time-variant hydrological parameters SC (indicating catchment active water storage capacity) and C (reflecting catchment evapotranspiration dynamics), whose average values increased 23.4% and 10.2%, respectively, due to drought. The change in the rainfall-runoff relationship identified by the data assimilation method is consistent with that arrived at by a statistical examination. The proposed method provides insights about the drivers of the changes in the rainfall-runoff relationship at the processes level. Declining groundwater and deep soil moisture depleted by persistent evapotranspiration of deep-rooted woody vegetation during drought are the main driving factors for the catchment behaviors change in the Wee Jasper catchment. The new method proposed in this study was found to be an effective technique for detecting both the change of hydrological behaviors induced by prolonged drought and its driving factors at the process level.

Wildfires are a cause of soil water repellency (hydrophobicity), which reduces infiltration while increasing erosion and flooding from post-fire rainfall. Post-fire soil water repellency degrades over time, often in response to repeated wetting and drying of the soil. However, in mountainous fire-prone forests such as those in the Western USA, the fire season often terminates in a cold and wet winter, during which soils not only wet and dry, but also freeze and thaw. Little is know about the effect of repeated freezing and thawing of soil on the breakdown of post-fire hydrophobicity. This study characterized the changes in hydrophobicity of Sierra Nevada mountain soils exposed to different combinations of wet-dry and freeze-thaw cycling. Following each cycle, hydrophobicity was measured using the Molarity of Ethanol test. Hydrophobicity declined similarly across all experiments that included a wetting cycle. Repeated freezing and thawing of dry soil did not degrade soil water repellency. Total soil organic matter content was not different between soils of contrasting hydrophobicity. Macroscopic changes such as fissures and cracks were observed to form as soil hydrophobicity decayed. Microscopic changes revealed by scanning electron microscope imagery suggest different levels of soil aggregation occurred in samples with distinct hydrophobicities, although the size of aggregates was not clearly correlated to the change in water repellency due to wet-dry and freeze-thaw cycling. A nine year climate and soil moisture record from Providence Critical Zone Observatory was combined with the laboratory results to estimate that hydrophobicity would persist an average of 144 days post-fire at this well-characterized, typical mid-elevation Sierra Nevada site. Most of the breakdown in soil water repellency (79%) under these climate conditions would be attributable to freeze-thaw cycling, underscoring the importance of this process in soil recovery from fire in the Sierra Nevada.

Stream thermal regimes are critical to the stability of freshwater habitats. There is growing concern that climate change will result in stream warming due to rising air temperatures, decreased shading in forested areas due to wildfires, and changes in streamflow. Groundwater plays an important role in controlling stream temperatures in mountain headwaters, where it makes up a considerable portion of discharge. This study investigated the controls on the thermal regime of a headwater stream, and the surrounding groundwater processes, in a catchment on the eastern slopes of the Canadian Rocky Mountains. Groundwater discharge to the headwater spring is partially sourced by a seasonal lake. Spring, stream, and lake temperature, water level, discharge and chemistry data were used to build a conceptual model of the system. Meteorological data was used to set up a stream temperature model. A tracer test was carried out to estimate hyporheic exchange along the study reach. This study presents a unique example of an indirectly lake-headed stream i.e., where the interaction of groundwater and lake water, and the hydraulic gradient determine the resulting stream temperature. Energy balance of the stream is mainly controlled by radiation. Sensible and latent heat fluxes play a secondary role, but their effects generally cancel out. Hyporheic exchange is present but plays only a minor role in the energy balance. During snowfall events, the latent heat associated with melting of direct snowfall onto the water surface was responsible for rapid stream cooling. An increase in advective inputs from groundwater and hillslope pathways did not result in observed cooling of stream water during rainfall events. The results from this study will assist water resource and fisheries managers in adapting to stream temperature changes under a warming climate.

Increased urbanization, coupled with the projected impacts of climatic change, mandates further evaluation of the impact of urban development on water flow paths to guide sustainable land use planning. Though the general urbanization impacts of increased storm runoff peaks and reduced baseflows are well known; how the complex, non-stationary interaction of the dominant water fluxes within dynamic urban water stores sustain streamflow regimes over longer periods of time are less well quantified. In particular, there is a challenge in how hydrological modelling should integrate the juxtaposition of rapid and slower flow pathways of the urban ‘karst’ landscape and different approaches need evaluation. In this context, we utilized hydrological and water stable isotope datasets within a modelling framework that combined the commonly used HEC urban runoff model along with a simple hydrological tracer module and transit time modelling to evaluate the spatial and temporal variation of water flow paths and ages within a heavily urbanized 217km 2 catchment in Berlin, Germany. Deeper groundwater was the primary flow component within less urbanized regions of the catchments, with increased direct runoff and shallow subsurface contributions in more urbanized areas near the catchment outlet. The addition of wastewater effluent in the mid-reaches of the catchment was the dominant water supply to the lower stream, and sustained baseflows during the summer months. Water ages from each modelling approach imitated flow contributions and opportunity for mixing with subsurface storage; with older water and lower young water contributions in less urbanized sub-catchments and younger water and higher young water contributions in more urbanized regions. The results form a first step towards more integrated modelling tools for similar peri-urban catchments, given the potential limitations of more simple model frameworks. The results have broader implications for assessing the uncertainty in evaluating urban impacts on hydrological function under environmental change.

The hydraulic properties of coastal aquifer systems are relevant to various hydrogeological, hydro-ecological and engineering problems. This study presents an analytical solution for predicting groundwater head fluctuations induced by dual-tide in multilayered island aquifer systems, consisting of an unconfined aquifer on the top and any number of leaky aquifers below. The solution was derived via the methods of matrix differential calculus and separation of variables. It is more general than any existing analytical solutions for the tidal pressure propagation since the new solution can consider multilayered aquifer systems along with the effects of leakage and aquifer length. Using this solution, we illustrated potential errors that may occur due to neglecting one or more vital factors affecting groundwater fluctuations. Besides, we articulated the groundwater response to the dual-tide in complex coastal aquifers. Considering that some thin semipermeable layers may be ignored in practical field investigation, we also demonstrated the effects due to simplification of aquifer layers. The results showed that with the increase in the number of overlapped leaky layers, the tidal propagation in the bottom part of multilayered aquifer system approaches that in a single confined aquifer with the same transmissivity and storage.