Indrani Pal

The future role of dams in the United States of America

Storage and controlled distribution of water have been key elements of a human strategy to overcome the space and time variability of water, which have been marked by catastrophic droughts and floods throughout the course of civilization. In the United States, the peak of dam building occurred in the mid-20th century with knowledge limited to the scientific understanding and hydrologic records of the time. Ecological impacts were considered differently than current legislative and regulatory controls would potentially dictate. Additionally, future costs such as maintenance or removal beyond the economic design life were not fully considered. The converging risks associated with aging water storage infrastructure and uncertainty in climate in addition to the continuing need for water storage, flood protection, and hydropower result in a pressing need to address the state of dam infrastructure across the nation. Decisions regarding the future of dams in the United States may, in turn, influence regional water futures through groundwater outcomes, economic productivity, migration, and urban growth. We advocate for a comprehensive national water assessment and a formal analysis of the role dams play in our water future. We emphasize the urgent need for environmentally and economically sound strategies to integrate surface and groundwater storage infrastructure in local, regional, and national water planning considerations. A research agenda is proposed to assess dam failure impacts and the design, operation, and need for dams considering both paleo and future climate, utilization of groundwater resources, and the changing societal values toward the environment.

Modeling winter rainfall in Northwest India using a hidden Markov model: understanding occurrence of different states and their dynamical connections

Abstract A multiscale-modeling framework for daily rainfall is considered and diagnostic results are presented for an application to the winter season in Northwest India. The daily rainfall process is considered to follow a hidden Markov model (HMM), with the hidden states assumed to be an unknown random function of slowly varying climatic modulation of the winter jet stream and moisture transport dynamics. The data used are from 14 stations over Satluj River basin in winter (December–January–February–March). The period considered is 1977/78–2005/06. The HMM identifies four discrete weather states, which are used to describe daily rainfall variability over study region. Each state was found to be associated with a distinct atmospheric circulation pattern, with the driest and drier states, State 1 and 2 respectively, characterized by a lack of synoptic wave activity. In contrast, the wetter and wettest states, States 3 and 4 respectively, are characterized by a zonally oriented wave train extending across Eurasia between 20N and 40N, identified with ‘western disturbances’ (WD). The occurrence of State 4 is strongly conditioned by the El Nino and Indian Ocean Dipole (IOD) phenomena in winter, which is demonstrated using large-scale correlation maps based on mean sea level pressure and sea surface temperature. This suggests that there is a tendency of higher frequency of the wet days and intense WD activities in winter during El Nino and positive IOD years. These findings, derived from daily rainfall station records, help clarify the sequence of Northern Hemisphere mid-latitude storms bringing winter rainfall over Northwest India, and their association with potentially predictable low frequency modes on seasonal time scales and longer.

Patterns of hydrological drought indicators in major U.S. River basins

Climate non-stationarity, changes in land use and water management practices affect regional hydrological extremes. This research considers extreme low streamflow (“low flow” henceforth) and flow deficit characteristics, which are important indicators of water scarcity within the broader context of droughts. The results indicate that the number of stations with positive significant trends in low flows is generally higher than the number of stations with negative significant trends; however, the fraction of total number of stations with negative trends has been increasing in the last four decades while the fraction of total number of stations with positive trends have been decreasing, making a gradual reduction in the wetting tendency in low flows in the conterminous U.S. Regional differences in low flow trends are notable, which echo the local climatic changes and soil moisture trends documented in the recent National Climate Assessment (NCA), as well as the changes in cropping and irrigation practices, and creation of forest plantations. A reversal of trend is seen for some northern regions since 1980s. Patterns in return periods and corresponding return values of low flows are also examined, which suggests changing risk conditions that are important for water-resources decision-making. Persistent low flow conditions in a river can lead to chronic water scarcity—a main driver of societal and cross-boundary conflicts. Thus, this research identifies “hotspot” locations where suitable adaptive management measures are most needed.