Description: Accurate quantification of evaporation from small water storages is essential for water management and planning, particularly in water-scarce regions. In order to ascertain suitable methods for direct measurement of evaporation from small water bodies, this study presents a comparison of eddy covariance and scintillometry measurements from a reservoir in southeast Queensland, Australia. The work presented expands on a short study presented by McJannet et al. (2011) to include comparisons of eddy covariance measurements and scintillometer derived predictions of surface energy fluxes under a wide range of seasonal weather conditions. In this study analysis was undertaken to ascertain whether important theoretical assumptions required for both techniques are valid in the complex environment of a small reservoir. Statistical comparison, energy balance closure and the relationship between evaporation measurements and key environmental controls were used to compare the results of the two techniques. Reasonable agreement was shown between the sensible heat flux measurements from eddy covariance and scintillometry, while scintillometer derived estimates of latent heat flux were approximately 21 % greater than eddy covariance measurements. We suggest possible reasons for this difference and provide recommendations for further research for improving measurements of surface energy fluxes over small water bodies using EC and scintillometry.

Description: We explore the possibility of using remotely-sensed soil moisture data and in-situ discharge observations to calibrate a large-extent hydrological model. The model used is PCR-GLOBWB-MOD, which is a physically-based and fully-coupled groundwater-land surface model operating at a daily basis and having a resolution of 30 arc-second (about 1 km at the equator). As a test bed, we use the combined Rhine-Meuse basin (total area: about 200 000 km 2 ), where there are 4250 point-scale observed groundwater head time series that are used to verify the model results. Calibration is performed by simulating 3045 model runs with varying parameter values affecting groundwater head dynamics. The simulation results of all runs are evaluated against the remotely sensed soil moisture time series of SWI (Soil Water Index) and field discharge data. The former is derived from European Remote Sensing scatterometers and provides estimates of the first meter profile soil moisture content at 30 arc-minute resolution (50 km at the equator). From the evaluation of these runs, we then introduce a step-wise calibration approach that considers stream discharge first, then soil moisture, and finally verify the resulting simulation to groundwater head observations. Our results indicate that the remotely sensed soil moisture data can be used for the calibration of upper soil hydraulic conductivities determining simulated groundwater recharge of the model. However, discharge data should be included to obtain full calibration of the coupled model, specifically to constrain aquifer transmissivities and runoff-infiltration partitioning processes. The step-wise approach introduced in this study, using both discharge and soil moisture data, can calibrate both discharge and soil moisture, as well as predicting groundwater head dynamics with acceptable accuracy. As our approach to parameterize and calibrate the model uses globally available data sets only, it opens up the possibility to set up large extent coupled groundwater-land surface models in other basins or even globally.

Description: A new Lagrangian particle-based method is presented to simulate reactive transport in natural porous media. This technique is based on Modified Moving Particle Semi-implicit (MMPS) and takes as input high-resolution voxel images of natural porous media. The flow field in the medium is computed by solving the incompressible Navier-Stokes equations. Moreover, a multi-component ion transport model is coupled with a homogeneous and heterogeneous reactions module to handle pore-space alteration (i.e., pore-wall dissolution). The model is first successfully validated against the experimental data available in the literature. Subsequently, x-ray micro-tomographic images of two naturally-occurring porous media are used to investigate the impact of reaction kinetics and porespace topology on pore-space alteration induced by brine acidification in subsurface conditions. We observed that at the normal rates of reactions no significant change in porosity and permeability takes place in the short term. Whereas, higher reaction rates caused major changes in the macroscopic properties (e.g., porosity and permeability) of the rocks. We also show that these changes are strongly affected by the rocks’ pore-scale topologies.

Description: General circulation models (GCMs) provide reliable simulations of global and continental scale atmospheric variables, yet have limited skill in simulating variables important for water resource management at regional to catchment scales. GCM simulations suffer from a range of uncertainties leading to transient (changing over time) and systemic (consistent over time) biases in the output when compared to observed records. An important GCM bias in managing water resources infrastructure is the under-representation of interannual variability, or persistence, a characteristic central to the simulation of floods and droughts. This study presents a performance metric, the aggregated persistence score (APS), which is used to assess the reliability of GCMs in simulating precipitation persistence. The APS identifies regions where GCMs poorly represent the amount of interannual variability seen in the observed precipitation. This study calculated the APS at monthly aggregations for GCM precipitation as well as GCM precipitation that was bias corrected to better represent low-frequency variability. It was found that there were (1) large spatial variations in the skill of GCMs to capture observed rainfall persistence, (2) widespread under-simulation of rainfall persistence characteristics in GCMs, and (3) substantial improvement in rainfall persistence after applying bias correction.

Description: [1] Statistical hydraulic models predict the frequency distributions of point hydraulic variables, relative to their reach-averaged values, in a stream reach based on its average characteristics (e.g., discharge, depth, width, average particle size). The models initially developed in Europe have not been tested for steeper streams (〉 4%) with coarse grain size. We recorded water velocities and depths in 44 reaches of steep streams in tropical islands and the Alps during 69 surveys. We fitted the observed distributions of velocities and depths using a mixture of two distributions, one with low variance and the other with a high variance. Then, we predicted the mixing parameter on the basis of the reach-averaged characteristics. We compared the observed and predicted frequencies for five classes of velocities, including a class of negative velocities, and four classes of water depths. The predictions of class frequencies have a bias of ≤ 5%. Our statistical model of velocity distribution predicts the frequencies of velocity classes with an explained variance between 33-72% for four classes of velocity and null for a class of intermediate velocity. The statistical model of depth distributions was less efficient with an explained variance between 25-38% for three classes of depth and null for large depths. The average Froude number, the total height of large drops relative to the reach length and the average slope are the main explanatory variables of velocity and depth distributions.

Description: Hyporheic fluxes influence ecological processes across a continuum of timescales. However, few studies have been able to characterize hyporheic fluxes and residence time distributions (RTDs) over timescales of days to years, during which evapotranspiration (ET) and seasonal flood pulses create unsteady forcing. Here we present a data-driven, particle-tracking piston model that characterizes hyporheic fluxes and RTDs based on measured vertical head differences. We used the model to test the relative influence of ET and seasonal flood pulses in the Everglades (FL, USA), in a manner applicable to other low-energy floodplains or broad, shallow streams. We found that over the multi-year timescale, flood pulses that drive relatively deep (~1 m) flow paths had the dominant influence on hyporheic fluxes and residence times but that ET effects were discernible at shorter timescales (weeks to months) as a break in RTDs. Cumulative RTDs on either side of the break were generally well represented by lognormal functions, except for when ET was strong and none of the standard distributions applied to the shorter timescale. ET increased hyporheic fluxes by 1-2 orders of magnitude at the monthly timescale and decreased six-year mean residence times by 53-87%. Long, slow flow paths driven by flood pulses increased six-year hyporheic fluxes by another 1-2 orders of magnitude, to a level comparable to that induced over the short term by shear flow in streams. Results suggest that models of intermediate-timescale processes should include at least two storage zones with different RTDs, and that supporting field data collection occur over 3-4 years.

Description: Variability in sediment hydraulic properties associated with landscape depositional and erosional features can influence groundwater recharge processes by affecting soil-water storage and transmission. This study considers recharge to aquifers underlying river-incised glaciated terrain where the distribution of clay-rich till is largely intact in upland locations but has been removed by alluvial erosion in stream valleys. In a stream-dissected glacial region in eastern Nebraska (Great Plains region of the United States), recharge estimates were developed for nested profile, aquifer, and regional scales using unsaturated zone profile measurements (matric potentials, Cl - and 3 H), groundwater tracers (CFC-12 and SF 6 ), and a remote sensing-assisted water balance model. Results show a consistent influence of till lithology on recharge rates across nested spatial scales despite substantial uncertainty in all recharge estimation methods, suggesting that minimal diffuse recharge occurs through upland glacial till lithology whereas diffuse recharge occurs in river valleys where till is locally absent. Diffuse recharge is estimated to account for a maximum of 61% of total recharge based on comparison of diffuse recharge estimated from the unsaturated zone (0-43 mm yr -1 ) and total recharge estimated from groundwater tracers (median 58 mm yr -1 ) and water balance modeling (median 56 mm yr -1 ). The results underscore the importance of lithologic controls on the distributions of both recharge rates and mechanisms.

Description: Processes of infiltration, transport and outflow in unsaturated soil necessarily involve the dissipation of energy through various processes. Accounting for these energetic processes can contribute to modelling hydrological and ecological systems. The well-documented hysteretic relationship between matric potential and moisture content in soil suggests that one such mechanism of energy dissipation is associated with the cycling between wetting and drying processes, but it is challenging to estimate the magnitude of the effect in situ . The Preisach model, a generalisation of the Independent Domain model, allows hysteresis effects to be incorporated into dynamical systems of differential equations. Building on earlier work using such systems with field data from the south-west of Ireland, this work estimates the average rate of hysteretic energy dissipation. Through some straightforward assumptions, the magnitude of this rate is found to be of O (10 − 5)Wm − 3.

Description: we propose a new method based on temperature time series of surface and streambed pore waters to monitor local changes in streambed surface elevations at a nominally daily time scale. The proposed method uses the naturally occurring daily temperature signal changes in amplitude and phase between stream water and the water flowing within the streambed sediment. Application of the method in a fine-bedded stream predicts the timing and magnitude of a prescribed sequence of scour and deposition. This provides a new, effective, easy to use, and economic methodology to monitor the temporal evolution of erosion and depositional patterns in rivers.