Hoori Ajami

ABSTRACT The degree of land surface-subsurface coupling is controlled by complex interactions between the atmosphere, land surface condition and subsurface hydrologic characteristics. Global climate models project increases in temperature and changes in precipitation rates and patterns which in turn alter terrestrial water and energy budgets impacting water resources. However, the degree of land surface-subsurface coupling under scenarios of land cover and climate change has not been fully explored. In this study, we used an integrated groundwater-surface water-land surface model (ParFlow.CLM) across a semi-arid catchment located in the central west New South Wales, Australia to assess variability in water and energy fluxes under historic condition and scenarios of climate and land cover change. The Baldry hydrological observatory situated in a topographically flat terrain has the area of 2 km2 and contains two distinct land cover types of pasture and a regenerated Eucalyptus forest. High resolution groundwater level measurements in the site reveal differences in groundwater connectivity in wet versus dry periods in pasture and Eucalyptus forest for the historic condition. Using downscaled climate forcing obtained from a regional climate model for eastern Australia, the degree of land surface-subsurface coupling within the catchment was examined under various scenarios of climate and changes in land cover types. It is expected that a fully integrated hydrologic model like ParFlow.CLM improve predictions in land-atmospheric feedback processes under changes in hydrologic conditions.

Quantifying spatial and temporal variability of riparian evapotranspiration (ET) is essential in water resources management especially in management and restoration of riparian ecosystems where multiple agricultural, industrial, and domestic users may exist. To enhance riparian evapotranspiration estimation in a MODFLOW groundwater model, RIPGIS-NET, an ArcGIS custom application, was developed to derive parameters and visualize results of spatially explicit riparian evapotranspiration in groundwater flow models for ecohydrology, riparian ecosystem management, stream restoration and water resources applications. RIPGIS-NET works with RIP-ET, a modeling package for MODFLOW. RIP-ET improves riparian ET simulations by using a set of eco-physiologically based ET curves for plant functional subgroups (PFSG), and is able to separate ground evaporation and plant transpiration processes. To evaluate impact of riparian restoration scenarios on groundwater resources, the above packages were applied to MODFLOW model of hypothetical Dry Alkaline Valley area. Using riparian ET curve files which show the relation between the groundwater level and ET, aerial extent of riparian vegetation in each season and a digital elevation map, RIPGIS-NET derived RIP-ET model parameters for each season. After running MODFLOW, groundwater head dynamics and spatial variability of riparian ET were visualized in GIS environment for each restoration scenario. This study provided useful information for riparian restoration planning in this area. It further highlighted the advantage of using spatially explicit models and datasets for riparian restoration planning.

ABSTRACT Long-duration high-volume dam releases are unique anthropogenic events with no naturally occurring equivalents. The impact from such dam releases on a downstream Quaternary alluvial aquifer in New South Wales, Australia, is assessed. It is observed that long-duration (> 26 days), high-volume dam releases (> 8,000 ML/day average) result in significant variations in river-aquifer interactions. These variations include a flux from the river to the aquifer up to 6.3 m(3)/day per metre of bank (at distances of up to 330 m from the river bank), increased extent and volume of recharge/bank storage, and a long-term (> 100 days) reversal of river-aquifer fluxes. In contrast, during lower-volume events (< 2,000 ML/day average) the flux was directed from the aquifer to the river at rates of up to 1.6 m(3)/day per metre of bank. A groundwater-head prediction model was constructed and river-aquifer fluxes were calculated; however, predicted fluxes from this method showed poor correlation to fluxes calculated using actual groundwater heads. Long-duration high-volume dam releases have the potential to skew estimates of long-term aquifer resources and detrimentally alter the chemical and physical properties of phreatic aquifers flanking the river. The findings have ramifications for improved integrated management of dam systems and downstream aquifers.

ABSTRACT One of the main challenges in the application of coupled or integrated hydrologic models is specifying a catchment's initial conditions in terms of soil moisture and depth-to-water table (DTWT) distributions. One approach to reducing uncertainty in model initialization is to run the model recursively using either a single year or multiple years of forcing data until the system equilibrates with respect to state and diagnostic variables. However, such "spin-up" approaches often require many years of simulations, making them computationally intensive. In this study, a new hybrid approach was developed to reduce the computational burden of the spin-up procedure by using a combination of model simulations and an empirical DTWT function. The methodology is examined across two distinct catchments located in a temperate region of Denmark and a semi-arid region of Australia. Our results illustrate that the hybrid approach reduced the spin-up period required for an integrated groundwater–surface water–land surface model (ParFlow.CLM) by up to 50%. To generalize results to different climate and catchment conditions, we outline a methodology that is applicable to other coupled or integrated modeling frameworks when initialization from an equilibrium state is required.

ABSTRACT The degree of land surface-subsurface coupling is controlled by complex interactions between the atmosphere, land surface condition and subsurface hydrologic characteristics. Global climate models project increases in temperature and changes in precipitation rates and patterns which in turn alter terrestrial water and energy budgets impacting water resources. However, the degree of land surface-subsurface coupling under scenarios of land cover and climate change has not been fully explored. In this study, we used an integrated groundwater-surface water-land surface model (ParFlow.CLM) across a semi-arid catchment located in the central west New South Wales, Australia to assess variability in water and energy fluxes under historic condition and scenarios of climate and land cover change. The Baldry hydrological observatory situated in a topographically flat terrain has the area of 2 km2 and contains two distinct land cover types of pasture and a regenerated Eucalyptus forest. High resolution groundwater level measurements in the site reveal differences in groundwater connectivity in wet versus dry periods in pasture and Eucalyptus forest for the historic condition. Using downscaled climate forcing obtained from a regional climate model for eastern Australia, the degree of land surface-subsurface coupling within the catchment was examined under various scenarios of climate and changes in land cover types. It is expected that a fully integrated hydrologic model like ParFlow.CLM improve predictions in land-atmospheric feedback processes under changes in hydrologic conditions.

ABSTRACT The computational effort associated with physically based distributed hydrological models is one of their major limitations that restrict their application in soil moisture and land surface flux simulation problems for large catchments. In this work, a new approach for reducing the computational effort associated with such models is investigated. This approach involves the formation of equivalent cross-sections, designed in a manner that ensures comparable accuracy in simulating the hydrological fluxes as a fully distributed simulation. Single or multiple equivalent cross-sections are formulated in each Strahler's first order sub-basin on the basis of topographic and physiographic variables representing the entire or part of the sub-basin. An unsaturated soil moisture movement model based on a 2-dimensional solution of the Richards' equation is used for simulating the soil moisture and hydrologic fluxes. The equivalent cross-section approach and the model are validated against observed soil moisture data in a semi-arid catchment and found consistent. The results indicate that the equivalent cross-section approach is an efficient alternative for reducing the computational time of distributed hydrological modeling while maintaining reasonable accuracy in simulating hydrologic fluxes, in particular dominant fluxes such as transpiration and soil evaporation in semi-arid catchments.

Climate variability and change impact groundwater resources by altering recharge rates. In semi-arid Basin and Range systems, this impact is likely to be most pronounced in mountain system recharge (MSR), a process which constitutes a significant component of recharge in these basins. Despite its importance, the physical processes that control MSR have not been fully investigated because of limited observations and the complexity of recharge processes in mountainous catchments. As a result, empirical equations, that provide a basin-wide estimate of mean annual recharge using mean annual precipitation, are often used to estimate MSR. Here North American Regional Reanalysis data are used to develop seasonal recharge estimates using ratios of seasonal (winter vs. summer) precipitation to seasonal actual or potential evapotranspiration. These seasonal recharge estimates compared favorably to seasonal MSR estimates using the fraction of winter vs. summer recharge determined from isotopic data in the Upper San Pedro River Basin, Arizona. Development of hydrologically based seasonal ratios enhanced seasonal recharge predictions and notably allows evaluation of MSR response to changes in seasonal precipitation and temperature because of climate variability and change using Global Climate Model (GCM) climate projections. Results show that prospective variability in MSR depends on GCM precipitation predictions and on higher temperature. Lower seasonal MSR rates projected for 2050-2099 are associated with decreases in summer precipitation and increases in winter temperature. Uncertainty in seasonal MSR predictions arises from the potential evapotranspiration estimation method, the GCM downscaling technique and the exclusion of snowmelt processes.

ABSTRACT Groundwater recharge is likely to be altered as a result of climate change and variability impacting groundwater resources. In semi-arid Basin and Range systems where Mountain System Recharge (MSR) represents a significant component of recharge, this impact is likely to be more pronounced. Despite the importance of MSR in such basins' water budget, physical processes that control MSR have not been fully investigated due to complexity of recharge processes in mountainous catchments and limited soil moisture and water level elevation data. In most groundwater models, MSR is either derived from empirical relationships or estimated during the model calibration and water balance analysis. Therefore, these models are not capable of assessing the impact of climate variability and change on groundwater resources. The objective of this research is to enhance our conceptual understanding of MSR, and quantify temporal and spatial variability of MSR in selected semi-arid catchments in the Basin and Range province of Arizona. Water budget analysis was performed on a seasonal time scale using the Soil and Water Assessment Tool (SWAT2005). Isotopic and soil moisture data were used to provide a constraint on recharge seasonality and water balance partitioning. Preliminary results show annual variability of MSR with pronounced differences in winter and summer seasons. The ratio of MSR to precipitation varied between (0-20%) in summer with a median of 8% compared to (0-50%) in winter with a median of 18%. Moreover, a threshold response of MSR to winter and summer precipitation and soil moisture was shown over the simulation period in different catchments. These results demonstrate the advantage of using modeling approaches that can evaluate these seasonal recharge thresholds. The results further highlight the need for further understanding of the physical factors in semi-arid catchments that control precipitation partitioning into MSR such as vegetation, soil type and slope.

ABSTRACT Quantification of groundwater flow dynamics and of the interactions among groundwater, surface water, and riparian vegetation, represent key components in the development of a balanced restoration plan for functional riparian ecosystems. A groundwater model was developed using MODFLOW 2000 to support of riparian restoration along the Colorado River Delta (Mexico: Baja California, Sonora). The Colorado River is widely recognized as one of the most modified and allocated rivers in the United States. For over 50 years flows into the Delta were severely reduced by the requirements of an emergent American West. However, subsequent to discharge pulses associated with the filling of Lake Powell, and the increased precipitation that accompanied ENSO cycles, a semblance of a native riparian habitat has been observed in the Delta since the 1980's (Zamora- Arroyo et al. 2001). The Delta and the riparian ecosystems of the region have since become the focus of a substantial body of multidisciplinary research. The research goal is to understand water table dynamics with particular attention to stream-aquifer interactions and groundwater behavior in the root zone. Groundwater reliant transpiration requirements were quantified for a set of dominant native riparian species using the Riparian ET (RIP-ET) package, an improved MODFLOW evapotranspiration (ET) module. RIP-ET simulates ET using a set of eco-physiologically based curves that more accurately represents individual plant species, reflects habitat complexity, and deals spatially with plant and water table distribution. When used in conjunction with a GIS based postprocessor (RIP-GIS.net), RIP-ET provides the basis for mapping groundwater conditions as they relate to user-specified plant groups. This explicit link between groundwater and plant sustainability is a driver to restoration design and allows for scenario modeling of various hydrologic conditions. Groundwater requirements determined in this research will be used by the international non-profit organizations, The Sonoran Institute and Environmental Defense, to implement large-scale planting activities within the riparian corridor, and to secure instream flow rights for the Colorado River Delta from the Mexican Government.

Quantifying spatial and temporal variability of riparian evapotranspiration (ET) is essential in water resources management especially in management and restoration of riparian ecosystems where multiple agricultural, industrial, and domestic users may exist. To enhance riparian evapotranspiration estimation in a MODFLOW groundwater model, RIPGIS-NET, an ArcGIS custom application, was developed to derive parameters and visualize results of spatially explicit riparian evapotranspiration in groundwater flow models for ecohydrology, riparian ecosystem management, stream restoration and water resources applications. RIPGIS-NET works with RIP-ET, a modeling package for MODFLOW. RIP-ET improves riparian ET simulations by using a set of eco-physiologically based ET curves for plant functional subgroups (PFSG), and is able to separate ground evaporation and plant transpiration processes. To evaluate impact of riparian restoration scenarios on groundwater resources, the above packages were applied to MODFLOW model of hypothetical Dry Alkaline Valley area. Using riparian ET curve files which show the relation between the groundwater level and ET, aerial extent of riparian vegetation in each season and a digital elevation map, RIPGIS-NET derived RIP-ET model parameters for each season. After running MODFLOW, groundwater head dynamics and spatial variability of riparian ET were visualized in GIS environment for each restoration scenario. This study provided useful information for riparian restoration planning in this area. It further highlighted the advantage of using spatially explicit models and datasets for riparian restoration planning.

ABSTRACT Water managers are increasingly concerned about the potential impact of climate variability and change on groundwater resources. Climate impacts on groundwater resources are primarily determined by altering the amount of recharge and evapotranspiration (ET). Typically, groundwater models employ temporally static recharge or ET rates with limited spatial variability across the basin. As a result most groundwater models cannot be used to assess the impacts of climate on groundwater resources. A primary challenge addressing this shortcoming is the need for spatially and temporally explicit recharge and ET model inputs. Geographic Information Systems (GIS) and spatially explicit data can be applied to develop these improved model inputs by quantifying and distributing recharge and ET across the model domain. Two ArcGIS desktop applications were developed for ArcGIS 9.2 to enhance recharge and ET estimation- Arc- Recharge and RIPGIS-NET. Arc-Recharge an ArcGIS 9.2 custom application is developed to quantify and distribute recharge along MODFLOW cells. Using spatially explicit precipitation data and Digital Elevation Model (DEM), Arc-Recharge routes water through the landscape and distributes the recharge to the appropriate groundwater model cells. RIPGIS-NET is an ArcGIS custom application that was developed to provide parameters for the RIP-ET package. RIP-ET is an improved MODFLOW ET module that simulates ET using a set of eco-physiologically based ET curves. RIPGIS-NET improves alluvial recharge estimation by providing spatially explicit information about the riparian/wetland ET. Application of Arc-Recharge and RIPGIS-NET in groundwater modeling enhances recharge and ET estimation by incorporating temporally and spatially explicit data. Using such tools, assessment of climate variability on groundwater resources will be enhanced.

Spatial and temporal variability of land surface fluxes are controlled by complex interactions among soil, vegetation and atmosphere. Despite the importance of groundwater dynamics on soil moisture distribution, especially in areas with shallow water table, the extent of land surface-subsurface interactions in catchments with various geologic and climate conditions are poorly understood. To examine the nature of these interactions and improve understanding of temporal and spatial variability of land surface fluxes in a ...