Konstantine P. Georgakakos

Analysis of Model-Calculated Soil Moisture over the United States (1931–1993) and Applications to Long-Range Temperature Forecasts

Abstract A long time series of monthly soil moisture data during the period of 1931–1993 over the entire U.S. continent has been created with a one-layer soil moisture model. The model is based on the water budget in the soil and uses monthly temperature and monthly precipitation as input. The data are for 344 U.S. climate divisions during the period of 1931–1993. The main goals of this paper are 1) to improve our understanding of soil moisture and its effects on the atmosphere and 2) to apply the calculated soil moisture toward long-range temperature forecasts. In this study, the model parameters are estimated using observed precipitation, temperature, and runoff in Oklahoma (1960–1989) and applied to the entire United States. The comparison with the 8-yr (1984–1991) observed soil moisture in Illinois indicates that the model gives a reasonable simulation of soil moisture with both climatology and interannual variability. The analyses of the calculated soil moisture show that the climatological soil mois...

A Monte Carlo Study of Rainfall Sampling Effect on a Distributed Catchment Model

A Monte Carlo study of a physically based distributed-parameter hydrologic model is described. The catchment model simulates overland flow and streamflow, and it is based on the kinematic wave concept. Soil Conservation Service curves are used to model rainfall excess within the basin. The model was applied to the Ralston Creek watershed, a small (7.5 km2) rural catchment in eastern Iowa. Sensitivity of the model response with respect to rainfall-input spatial and temporal sampling density was investigated. The input data were generated by a space-time stochastic model of rainfall. The generated rainfall fields were sampled by the varied-density synthetic rain gauge networks. The basin response, based on 5-min increment input data from a network of high density with about 1 gauge per 0.1 km2, was assumed to be the “ground truth,” and other results were compared against it. Included in the study was also a simple lumped parameter model based on the unit hydrograph concept. Results were interpreted in terms of hydrograph characteristics such as peak magnitude, time-to-peak, and total runoff volume. The results indicate higher sensitivity of basin response with respect to the temporal resolution than to the spatial resolution of the rainfall data. Also, the frequency analysis of the flood peaks shows severe underestimation by the lumped model. This may have implications for the design of hydraulic structures.

Large-Scale Aspects of the United States Hydrologic Cycle

Abstract A large-scale, gridpoint, atmospheric, hydrologic climatology consisting of atmospheric precipitable water, precipitation, atmospheric moisture flux convergence, and a residual evaporation for the conterminous United States is described. A large-scale, basin, hydrologic climatology of the same atmospheric variables is also described, as well as residual surface water and streamflow divergence or runoff for various large-scale river basins terminating at the United States boundary. Climatologically, precipitation, which had a U.S. annual mean of more than 2.1 mm day−1, was largely balanced by evaporation; atmospheric moisture flux convergence was also an important contributor (∼0.5 mm day−1), especially during the wintertime, and especially along the U.S. west coast. At the surface, seasonal and anomalous surface water (including snow) variations on the order of 10 cm yr−1 were forced by seasonal variations of about 1 mm day−1 in atmospheric moisture flux convergence (precipitation minus evaporati...

Precipitation structure in the Sierra Nevada of California during winter

Influences of upper air characteristics along the coast of California upon winter-time (November-April) precipitation in the Sierra Nevada are investigated. Precipitation events in the Sierra Nevada region occur mostly during wintertime, irrespective of station location (leeside or windside) and elevation. Most precipitation episodes in the region are associated with moist southwesterly winds (coming from the southwest direction) and also tend to occur when the 700-mbar temperature at the upwind direction is close to −2°C. This favored wind direction and temperature signify the importance of both moisture transport and orographic lifting in augmenting precipitation in the region. By utilizing the observed dependency of the precipitation upon the upper air conditions, a linear model is formulated to quantify the precipitation observed at different sites as a function of moisture transport. The skill of the model increases with timescale of aggregation, reaching more than 50% variance explained at an aggregation period of 5–7 days. This indicates that upstream air moisture transport can be used to estimate the precipitation totals in the Sierra Nevada region.

Estimating the Dimension of Weather and Climate Attractors: Important Issues about the Procedure and Interpretation

Abstract When the reconstruction of attractors from observables is sought, the Grassberger-Procaccia algorithm for estimating the correlation dimension is often used. An overview of recent developments concerning data requirements and algorithm performance is presented within. In view of these developments the significance of previously estimated dimensions of weather and climate attractors is discussed.

Evidence of Deterministic Chaos in the Pulse of Storm Rainfall

Abstract Rainfall data obtained by a highly sensitive raingage have been analyzed for the presence of strange attractors. Analysis of three storms that occurred in Cambridge, Massachusetts revealed, for each storm, the presence of a low-dimensional strange attractor with correlation dimension that was less than 4. The datasets consist of the discrete time series of the interarrival times of one-hundredth of a millimeter rainfall amounts. In all cases, the number of data points in the datasets was at least 3300, which makes the evidence of determinism in storm rainfall strong.

Assessment of Folsom lake response to historical and potential future climate scenarios: 1. Forecasting

Abstract In collaboration with operational forecast and management Agencies, an integrated forecast-control system is designed and applied to a major reservoir in California to evaluate the potential benefits of climate information for flood control, hydroelectric energy production, and low flow augmentation. In addition to retrospective studies involving the historical period 1964–1993, system simulations were performed for the future period 2001–2030, under a control and a 1% greenhouse-gas-increase scenario. This paper presents the forecast component formulation and validates ensemble 30-day reservoir-inflow forecasts under a variety of situations. The control component formulation and corresponding reservoir management results are presented in Yao and Georgakakos, this issue. The forecast component is based on ensemble flow forecasting. Quantiles of the distribution of climate-model precipitation simulations are used to select catchment-scale historical daily precipitation time series for the generation of an ensemble of daily reservoir-inflow by hydrologic models. Ensemble generation takes into consideration both atmospheric-forcing and hydrologic-model uncertainties. Principal conclusions of this paper are that the integrated system provides reliable ensemble inflow volume forecasts for the majority of the deciles of forecast frequency, and that the use of climate model simulations is beneficial mainly during high flow periods. It is also found that to maintain reliability for future climate periods, generation of ensemble inflow forecasts should use input time series that reflect potential sharp changes in precipitation amount.

Measurement and utilization of on-site soil moisture data

Programs for the on-site measurement of soil moisture in the USA are reviewed. These are regional and national measurement programs that may be useful for the verification of remotely sensed soil moisture estimates and for hydroclimatic studies. Location and type of measurement are described. A technique for the utilization of on-site data of soil moisture and discharge together with remotely sensed data of the surface soil moisture is proposed for the estimation of soil water content aggregated over large areas. The technique is based on large-scale conceptual hydrologic models and on state estimation techniques that allow explicit account to be taken of measurement uncertainty. The proposed approach is explored in an example formulation applied to a 40 year record of monthly data from a 4672 km2 natural catchment in Illinois. This study shows the ability of simple conceptual hydrologic models to simulate well both flow and soil water in humid areas and with monthly data. It is further shown that, even when the remotely sensed measurements of the surface soil moisture carry substantial measurement errors, inference of lower soil water and of total soil water is possible with an expected error that is smaller than that achieved without the use of the remotely sensed data.

Impacts of climate variability on the operational forecast and management of the Upper Des Moines River Basin

Data from the regulated 14,000 km2 upper Des Moines River basin and a coupled forecast-control model are used to study the sensitivity of flow forecasts and reservoir management to climatic variability over scales ranging from daily to interdecadal. Robust coupled forecast-control methodologies are employed to minimize reservoir system sensitivity to climate variability and change. Large-scale hydrologic-hydraulic prediction models, models for forecast uncertainty, and models for reservoir control are the building blocks of the methodology. The case study concerns the 833.8 × 106 m3 Saylorville reservoir on the upper Des Moines River. The reservoir is operated by the U.S. Corps of Engineers for flood control, low-flow augmentation, and water supply. The total record of 64 years of daily data is divided into three periods, each with distinct characteristics of atmospheric forcing. For each climatic period the coupled forecast-control methodology is simulated with a maximum forecast lead time of 4 months and daily resolution. For comparison, the results of operation using current reservoir control practices were obtained for the historical periods of study. Large differences are found to exist between the probabilistic long-term predictions of the forecast component when using warm or cool and wet or dry initial conditions in the spring and late summer. Using ensemble input corresponding to warm or cool and wet or dry years increases these differences. Current reservoir management practices cannot accommodate historical climate variability. Substantial gain in resilience to climate variability is shown to result when the reservoir is operated by a control scheme which uses reliable forecasts and accounts for their uncertainty. This study shows that such coupled forecast-control decision systems can mitigate adverse effects of climatic forcing on regional water resources.

On the Design of National, Real-Time Warning Systems with Capability for Site-Specific, Flash-Flood Forecasts

Abstract Presented are necessary requirements of a modern-day flash-flood warning system that is capable of site-specific forecasts and that is suitable for national implementation. The requirements are identified based on the hydrometeorological character of the flash-flood phenomenon and on the real-time nature of the forecast procedure. Contemporary theories of heavy-rainfall and runoff generation and development are reviewed.