M. L. Kavvas

A regional-scale land surface parameterization based on areally-averaged hydrological conservation equations

Abstract In order to account for subgrid-scale spatial variability (heterogeneity) of land surface characteristics in regional-scale hydrological-atmospheric models, a land surface parameterization of areally-averaged sensible heat and evapotranspiration fluxes which is based upon areally-averaged hydrological soil water flow and soil heat flow equations, was developed. This land surface parameterization is fully coupled in a two-way interaction with the atmospheric boundary layer and the regional atmospheric models first layer. The Monin-Obukhov similarity theory which is utilized in modelling the atmospheric boundary layer, and the areallyaveraged hydrological conservation equations are strictly valid only over stationary-heterogeneous areas where the fluctuations of the hydrological and boundary layer state variable values, parameter values, and of boundary conditions have spatially invariant means, spatially invariant higher moments and spatially invariant probability distributions. Therefore, the co...

Physically Based Estimation of Maximum Precipitation over American River Watershed, California

A methodology for maximum precipitation (MP) estimation that uses a physically based numerical atmospheric model is proposed in this paper. As a case study, the model-based 72-h MP was estimated for the American River watershed (ARW) in California for the December 1996–January 1997 flood event. First, a regional atmospheric model, MM5, was calibrated and validated for the December 1996–January 1997 historical major storm event for the ARW, on the basis of the U.S. National Center for Atmospheric Research (NCAR) reanalysis data to demonstrate the model capability during the historical period. Then, the model-simulated historical storm event was maximized by modifying its boundary conditions. The model-simulated precipitation field in the ARW was successfully validated at nine individual rain gauge stations in the watershed. The computed basin-averaged precipitation was somewhat higher than observations obtained by the spatial interpolation of the rain gauge observations. This result suggests a limitation o...

Use of Green-Ampt model for analyzing one-dimensional convective transport in unsaturated soils

Abstract A general analytical solution to the equation describing purely convective vertical transport of a conservative solute under transient water flow conditions is presented. Richards equation does not yield analytical solutions for the flow field, except under restrictive choices of hydraulic properties relating water content, hydraulic conductivity and the soil matric potential. The utility of the Green-Ampt model for homogeneous unsaturated soils is investigated as an alternative to Richards equation. This model, though approximate, provides analytical solutions to the flow field. It is found that the flow quantities required for the analytical solution of convective transport (specifically the soil water storage) are predicted accurately by the Green-Ampt model. Comparisons of numerical and analytical solutions of solute concentration profiles suggest that the Green-Ampt water flow model is adequate for convective solute transport predictions, and that the flow field need not be computed by the more numerically expensive Richards equation for this problem.

WEHY-HCM for Modeling Interactive Atmospheric-Hydrologic Processes at Watershed Scale. I: Model Description

AbstractAmong the key problems in atmospheric and hydrologic sciences are the modeling of the interaction between the atmosphere and land surface hydrology while also quantifying the surface/subsurface hydrologic flow processes both in vertical and lateral directions, and modeling the heterogeneity in surface and subsurface hydrologic processes. Meanwhile, in standard water resources engineering practice, the planning and management of the water resources is performed over the geographical region of a watershed. To address these issues, a model of coupled atmospheric-hydrologic processes at the watershed scale, the Watershed Environmental Hydrology Hydro-Climate Model (WEHY-HCM), has been developed. The atmospheric model PSU/NCAR MM5 (Fifth Generation Mesoscale Model) was coupled to the watershed hydrology model WEHY through the atmospheric boundary layer to form the WEHY-HCM. The WEHY-HCM is especially useful for producing nonexistent atmospheric data as input to the modeling of surface and subsurface hy...

Physically Based Estimation of Maximum Precipitation over Three Watersheds in Northern California: Atmospheric Boundary Condition Shifting

AbstractMaximum precipitation during a historical period is estimated by means of a physically based regional atmospheric model over three watersheds in Northern California: the American River watershed (ARW), the Yuba River watershed (YRW), and the Upper Feather River watershed (UFRW). In Northern California, severe storm events are mostly caused by a high-moisture atmospheric flow from the Pacific Ocean, referred to as atmospheric river (AR). Therefore, a method to maximize the contribution of an AR on precipitation over each of the targeted watersheds is proposed. The method shifts the atmospheric boundary conditions of the regional atmospheric model in space with latitude and longitude so that the AR strikes each of the targeted watersheds in an optimal direction and location to maximize the precipitation over these watersheds. For this purpose, the fifth generation Penn State/National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5) is used as the regional atmospheric model, and the NCAR/...

Atmospheric Model-Based Streamflow Forecasting at Small, Mountainous Watersheds by a Distributed Hydrologic Model: Application to a Watershed in Japan

In this study, an experimental 12-h lead-time flood forecasting methodology that combines the fifth generation mesoscale model (MM5) of the U.S. National Center of Atmospheric Research with the physically based, spatially distributed watershed environmental hydrology (WEHY) model is described and applied to the Shiobara Dam watershed in Japan in order to explore its utility. The Shiobara-Dam watershed is a mountainous steep-sloped watershed that has an area of 123  km2 . Meanwhile, the routine atmospheric assimilation data that are provided by the Japan Meteorological Agency (JMA) over Japan, have spatial resolution of 20 km and are at 12-h time intervals. In order to utilize the JMA’s atmospheric data at 12-h intervals as initial and boundary conditions for 12-h lead-time hourly precipitation forecast inputs to the WEHY model of Shiobara-Dam watershed for runoff forecasts, the MM5 nonhydrostatic atmospheric forecast model was chosen and nested inside the JMA’s data domain. The JMA’s atmospheric data were...

Coupled Regional Hydroclimate Model and Its Application to the Tigris-Euphrates Basin

To establish a basinwide water management plan for the Tigris-Euphrates (TE) watershed, it is necessary to perform rigorous water balance studies of the whole watershed—at least for critical historical drought and flood conditions and under various water resources development scenarios. Water-balance studies over the watershed require climatic and hydrologic data sets, corresponding to historical critical flood and drought periods, at fine time and spatial grid resolutions provide the necessary hydroclimatic information. The Regional Hydroclimate Model of the Tigris-Euphrates (RegHCM-TE) and the associated geographic information system (GIS) were developed to downscale large-scale atmospheric data sets over the TE watershed and to reconstruct the aforementioned climatic and land hydrologic data sets at fine spatial and time increments. In RegHCM-TE, the earth system over the TE watershed is modeled as a fully coupled system of atmospheric processes aloft coupled with the atmospheric boundary layer, land-s...

Application of Discrete Autoregressive Moving Average models for estimation of daily runoff

Abstract Daily precipitation has been successfully modeled by the Discrete Autoregressive Moving Average (DARMA) family of models. One of these models is used in the estimation of daily streamflows by means of a linear transfer function. The statistical properties of the chosen DARMA precipitation process and the first- and second-order statistics of the relationships between daily precipitation and the corresponding streamflow are used in the determination of the transfer function parameters. The constructed process is applied to a watershed located in Indiana. The model identification, the parameter estimation, and the diagnostic checking are illustrated. This transfer model has the merit that it couples the first- and second-order statistics of the rainfall input and runoff output. The model limitations stem from the stationarity of the DARMA models used for the description of daily rainfall sequences. This requires the division of the year into stationary seasons and introduces minor inaccuracies at the season crossings.

Probabilistic/ensemble forecasting: a case study using hydrologic response distributions associated with El Niño/Southern Oscillation (ENSO)

Abstract Due to the non-linear processes and interactions of the hydroclimatic system, a given hydroclimatic event such as the El Nino/Southern Oscillation (ENSO) can lead to a range of possible hydrologic system responses described by a probability distribution. This probability distribution changes in space and time reflecting the non-stationary behavior of the hydroclimatic system. An initial approach in quantifying the evolving probability distributions of hydrologic system response utilizes a physically based hemispheric hydrologic model, PBHHM, that incorporates the salient physics of the hydroclimatic system for the midlatitudes of the Northern Hemisphere. The state variables of the model include atmospheric temperature, atmospheric water content, quasi-geostrophic potential vorticity, land hydrologic water storage, and land/sea surface temperature. The model is structured in such a way that characteristics (e.g. sea surface temperature, geopotential anomalies, etc.) of a hydroclimatic event such as ENSO can be incorporated into the model as a forcing event. The hydrologic system response probability distribution is quantified, via the land hydrologic water storage state variable. As a case study, the hydrologic system response probability distributions of the western continental United States to both the El Nino and La Nina phases of ENSO have been simulated. One hundred realizations were run for each phase using random initial conditions for the state variables in order to reflect differing hydroclimatic conditions during the initiation and evolution of the forcing event. The probability distributions of hydrologic system response and their evolution in space and time are described using relative frequency histograms, cumulative distribution functions, and contour plots of frequency histogram categories. Simulation results of the hydrologic system response probability distribution associated with each phase of the ENSO phenomenon are presented which show a distinct response that varies in space and time. The influence of the number of realizations upon these distributions will be discussed along with a means of incorporating the distributions into a water resources planning scheme.

On the physics of droughts. II : Analysis and simulation of the interaction of atmospheric and hydrologic processes during droughts

Abstract In this study a simple general circulation model (GCM) is developed to investigate the interaction of atmospheric and hydrologic processes during droughts. Only the vertically and latitudinally averaged mean temperature and mean water vapor content over the strip of Earth between 30 and 50°N latitudes are considered as atmospheric state variables; only two hydrologic state variables, the water storage (through the water balance equation) and ground temperature, are considered for describing the behavior of the hydrologic system. The ocean temperature is specified as a function of season only. The coupling between the atmosphere and the Earths surface occurs through exchanges of thermal energy (radiation, sensible and latent heat fluxes) and of water (evapotranspiration and rain), which are parameterized functions of the four state variables of the atmospheric-hydrologic interactive system. Using this GCM, the mechanism leading to drought conditions is investigated. It is shown to be a non-linear positive feedback mechanism in the sense that it feeds on itself. Imposing a temperature wave over a geographical region (western USA in this study) for a short period of the order of few months induces geophysical conditions in that region which tend to reduce the hydrologic water storage for a period of time of the order of years (called ‘drought growth period’). These conditions consist initially of increased cloudiness over the region, which reduces the net radiative flux of energy available at the surface. This in turn reduces the evapotranspiration which is needed to achieve condensation conditions in the atmosphere. Consequently, a reduction in rainfall occurs. After the end of the drought growth period, the hydrologic system recovers the water storage mainly owing to decreased evapotranspiration (related to reduced water storage) while rainfall resumes its normal levels. The time duration necessary to return to the climatological average conditions of water storage (called ‘drought recovery period’) is proportional to the peak drought severity during the growth period.