The response of playa and sabkha hydraulics and mineralogy to climate forcing

Dry playa lakes and sabkhat often represent the terminus of large ground water flow systems and act as integrators of both upgradient (recharge) and downgradient discharge (evaporation). Ground water levels beneath playa/sabkha systems show a variety of surprising responses driven by large evaporation demands and chemical processes not typically encountered in more humid regions. When the water table is very close to the land surface, almost instantaneous rises can be observed with little observed change in either upgradient ground water recharge or potential evaporation. Conversely, when water tables are several meters below the playa surface, water table responses to interannual variability of recharge can be damped and lag significantly behind such changes. This review of the dynamics of shallow water tables in playa lakes and sabkhat discusses the pertinent hydraulic and solute processes and extracts a simple but comprehensive model based on soil physics for predicting the water table response to either upstream recharge changes or changes in potential evaporation at the playa/sabkha. Solutes and associated authigenic minerals are also shown to be important in discriminating both the causes and effects of water level fluctuations.     

Variations in the depth distribution of phosphorus in soil profiles and implications for model-based catchment-scale predictions of phosphorus delivery to surface waters

The PSYCHIC process-based model for predicting sediment and phosphorus (P) transfer within catchments uses spatial data on soil-P derived from the National Soil Inventory (NSI) data set. These soil-P values are based on bulked 0–15 cm depth and do not account for variations in soil-P with depth. We describe the depth distribution of soil-P (total and Olsen) in grassland and arable soils for the dominant soil types in the two PSYCHIC study catchments: the Avon and the Wye, UK. There were clear variations in soil-P (particularly Olsen-P) concentrations with depth in untilled grassland soils while concentrations of total-P were broadly constant within the plough layer of arable soils. Concentrations of Olsen-P in arable soils, however, exhibited maximum values near the soil surface reflecting surface applications of fertilisers and manures between consecutive ploughing events. When the soil-P concentrations for the surface soil (0–5 cm average) were compared to both the profile-averaged (0–15 cm) and the NSI (0–15 cm) values, those for the surface soil were considerably greater than those for the average 0–15 cm depth. Modelled estimates of P loss using the depth-weighted average soil-P concentrations for the 0–5 cm depth layer were up to 14% greater than those based on the NSI data set due to the preferential accumulation of P at the soil surface. These findings have important implications for the use of soil-P data (and other data) in models to predict P losses from land to water and the interpretation of these predictions for river basin management.     

On Creating Global Gridded Terrestrial Water Budget Estimates from Satellite Remote Sensing

The increasing availability and reliability of satellite remote sensing products [e.g., precipitation (P), evapotranspiration (ET), and the total water storage change (TWSC)] make it feasible to estimate the global terrestrial water budget at fine spatial resolution. In this study, we start from a reference water budget dataset that combines all available data sources, including satellite remote sensing, land surface model (LSM) and reanalysis, and investigate the roles of different non-satellite remote sensing products in closing the terrestrial water budget through a sensitivity analysis by removing/replacing one or more categories of products during the budget estimation. We also study the differences made by various satellite products for the same budget variable. We find that the gradual removal of non-satellite data sources will generally worsen the closure errors in the budget estimates, and remote sensing retrievals of P, ET, and TWSC together with runoff (R) from LSM give the worst closure errors. The gauge-corrected satellite precipitation helps to improve the budget closure (4.2–9 % non-closure errors of annual mean precipitation) against using the non-gauge-corrected precipitation (7.6–10.4 % non-closure errors). At last, a data assimilation technique, the constrained Kalman filter, is applied to enforce the water balance, and it is found that the satellite remote sensing products, though with worst closure, yield comparable budget estimates in the constrained system to the reference data. Overall, this study provides a first comparison between the water budget closure using the satellite remote sensing products and a full combination of remote sensing, LSM, and reanalysis products on a quasi-global basis. This study showcases the capability and potential of the satellite remote sensing in closing the terrestrial water budget at fine spatial resolution if properly constrained.     

World's Largest In Situ Thermal Desorption Project: Challenges and Solutions

This paper presents the largest In Situ Thermal Desorption (ISTD) project completed to date. The redevelopment of a former aerospace manufacturing facility adjacent to a commercial airport was the main driver, requiring relatively rapid reduction of several chlorinated volatile organic compounds (CVOC) in a 3.2‐acre source zone. The source zone was divided into four quadrants with differing treatment depths, heated simultaneously using a total of 907 thermal conduction heater wells. Five different depths were selected across the area, according to the depth of contaminant impact. Prior to implementation, a risk and optimization study led to placement of a vertical sheet‐pile wall around the treatment zone to minimize groundwater flow, and a pilot test of a novel direct‐drive method for installation of the heater casings. Because of a shallow water table, a layer of clean fill was placed over the treatment zone, and partial dewatering was necessary prior to heating. A network of vertical multiphase extraction wells and horizontal vapor extraction wells was used to establish hydraulic and pneumatic control and to capture the contaminants. The site was split into four decision units, each with a rigorous soil sampling program which included collecting a total of 270 confirmatory soil samples from locations with the highest pretreatment CVOC concentrations requiring reduction to below 1 mg/kg for each contaminant. Temperature monitoring and mass removal trends were used to trigger the sampling events. Eventually, a small area near the center of the site required the installation of four additional heaters before the soil goals were reached after 238 days of heating. The total energy usage for heating and treating the source area was 23 million kWh—slightly lower than the estimated 26.5 million kWh. Actual energy losses and the energy removal associated with the extracted steam were lower than anticipated. An estimated 13,400 kg (29,800 lbs) of CVOC mass was removed, and all soil goals were met. This paper presents the challenges associated with a project of this scale and describes the solutions to successfully complete the ISTD remedy.     

Effects of soil moisture aggregation on surface evaporative fluxes

The effects of small-scale heterogeneity in land surface characteristics on the large-scale fluxes of water and energy in the land-atmosphere system has become a central focus of many climatology research experiments. The acquisition of high resolution land surface data through remote sensing and intensive land-climatology field experiments (like HAPEX, FIFE, and BOREAS) has provided data to investigate the interactions between microscale land-atmosphere interactions and macroscale models. To determine the effect of small scale heterogeneities, the spatially averaged evaporative fraction is analytically derived for spatially variable soil moisture and soil-atmospheric controls on evaporation at low soil moisture. This average evaporative fraction is compared with the evaporative fraction determined using the spatially averaged soil moisture, as if from a lumped, or aggregated, land surface model. Results show that the lumped-model based evaporation will over estimate evaporation during periods of low atmospheric demands (early morning/late afternoon, Winter periods, etc.) and under estimate evaporation during periods of high demand (midday Summer periods.) The accuracy of using ‘effective’ parameters in lumped macroscale models depends on the variability of soil moisture and the sensitivity of the soil-vegetation system to low soil moisture.     

Analysis of coupled axial and lateral deformation of roots in soil

Plant roots can help to stabilise slopes. Existing analytical models to predict their mechanical contribution are however limited: they typically focus on the ultimate limit state, employ various empirical factors, and ignore much of the underlying root-soil interaction. A new model was developed based on large deflection Euler-Bernoulli elastic beam theory that can be used to study the mobilisation of root strength under various loading conditions (direct shear and pull-out). Both lateral and axial loading of the root by the soil were incorporated, based on existing methodologies for foundation piles (p-y and t-z curves). The model is able to take the key parameters into account (root biomechanical properties, root architectural properties, and soil properties) while remaining quick to solve using a numerical boundary value problem solver. The model was compared with experimental direct shear test data using various root analogues (rubber, plastic, and wood) in dry sand with various densities and effective stress levels and was able to accurately predict the measured shear force-displacement behaviour. Comparison with experimentally measured pull-out force-displacement curves using rubber and wooden root analogues with various architectures in dry and partially saturated sands was also satisfactory. In the future, this model can aid with addressing long-standing problems in the root-reinforcement community: quantifying the effect of (sequential) mobilisation of root strength in direct shear, the effect of the angle at which the root crosses a shear plane, the effect of root topology on root-reinforcement or the effect of root bending, and root shear shear forces on root-reinforcement.     

Preliminary Results from an Astrophysically Relevant Radiation Transfer Experiment

The results of a diffusive radiation transport experiment in a simple geometry are presented. The experiment depends primarily on two variables, the target density and the temperature drive, which are characterized well. The experiment is designed to verify and validate radiation transport in codes. The codes can then be used to model astrophysical systems. The results of the experiments are found to be in good agreement with simulation results.     

HydroBlocks: a field‐scale resolving land surface model for application over continental extents

Land surface spatial heterogeneity plays a significant role in the water, energy, and carbon cycles over a range of temporal and spatial scales. Until now, the representation of this spatial heterogeneity in land surface models has been limited to over simplistic schemes because of computation and environmental data limitations. This study introduces HydroBlocks – a novel land surface model that represents field‐scale spatial heterogeneity of land surface processes through interacting hydrologic response units (HRUs). HydroBlocks is a coupling between the Noah‐MP land surface model and the Dynamic TOPMODEL hydrologic model. The HRUs are defined by clustering proxies of the drivers of spatial heterogeneity using high‐resolution land data. The clustering mechanism allows for each HRU's results to be mapped out in space, facilitating field‐scale application and validation. The Little Washita watershed in the USA is used to assess HydroBlocks' performance and added benefit from traditional land surface models. A comparison between the semi‐distributed and fully distributed versions of the model suggests that using 1000 HRUs is sufficient to accurately approximate the fully distributed solution. A preliminary evaluation of model performance using available in situ soil moisture observations suggests that HydroBlocks is generally able to reproduce the observed spatial and temporal dynamics of soil moisture. Model performance deficiencies can be primarily attributed to parameter uncertainty. HydroBlocks' ability to explicitly resolve field‐scale spatial heterogeneity while only requiring an increase in computation of one to two orders of magnitude when compared with existing land surface models is encouraging – ensemble field‐scale land surface modelling over continental extents is now possible. Copyright © 2016 John Wiley & Sons, Ltd.