Water Resources and Stormwater Management under Changing Climate: Issues

The w'ater resources management process that involves the planning and operation of hydrosystems aimed at fulfilling multiple objectives ranging from efficient water supply to reliable hydropower generation is evolving under changing climate. While short-term planning relies on bi-monthly to seasonal forecasts, long-term planning requires projections of future hydroclimatic variables up to several years to estimate spatial and temporal variability and availability of water.

Descriptive (i.e., simulation) and prescriptive (i.e., optimization) models are routinely used to model and optimize hydrosystems, respectively. While the development of these models is possible with a deeper conceptualization and understanding of the systems, there remain several issues/challenges that need to be addressed to achieve optimal operations of hydrosystems under changing climate (Teegavarapu, 2010), and they include (i) lack of clear guidance on handling multi-model, multiple scenario GCM-based model projections of future hydroclimatic variables; (ii) limited studies that address the issues of future climate uncertainty and decision-makers’ perceptions attached to projected future climate; (iii) limited use of climate-specific information for the short-term adaptive operation of hydrosystems; (iv) lack of methods to leverage improvements in forecasting approaches and forecast skill based on advancements in w'eather monitoring to effectively manage water resource systems (e.g., reservoir systems); (v) limited application and testing of approaches that consider multiple objectives in the context of an evolving climate; (vi) lack of flexibility in operations of reservoir systems when changes are recommended to the standard operating curves (rule curves or standard operating policies); (vii) lack of approaches to incorporate uncertainties associated with economic indicators in water management models; and (viii) lack of methodologies that consider all aspects of climate change including non- stationarity for infrastructure design.

Water resource planners and stormwater management agencies have access to several resources that are created by the United States EPA (USEPA) to promote resilient water systems under the Creating Resilient Water Utilities (CRWU) initiative (www.epa.gov/crwu). The initiative provides several resources/tools for the planning and management of water resources systems for changing climate. These tools include (i) Climate Resilience Evaluation and Awareness Tool (CREAT), (ii) Adaptation Case Studies Map, and (iii) CRWU Adaptation Strategies Guide. The Fourth National Climate Assessment (NCA4) volumes I and II were completed in 2018 by the U.S. Global Change Research Program (USGCRP), an organization or a group established by a presidential initiative and mandated by the U.S. Congress in the Global Change Research Act (GCRA) of 1990. Comprising 13 federal agencies, the USGCRP conducts or uses research on global change and its impact on society. The volumes I and II of NCA4 deal with climate science and impacts, risks, and adaptation in the United States, respectively. Many state water management agencies utilize forecasts about natural climate variability cycles in their short- and long-term planning models.

Hydrologic design and stormwater management are intricately interlinked as the former is essential for the latter. Stormwater management in response to climate change impacts has received enormous attention from local and regional agencies in the U.S. The USEPA (2016) has provided several guidelines for the improvement of urban stormwater infrastructure to handle uncertain future climate changes. Climate change and adaptation strategies are developed considering extreme precipitation events in several cities across the U.S. The planning opportunities that are explored include improvement or rehabilitation of infrastructure to plan for future changes in climate (or referred to as rightsizing infrastructure), rainwater harvesting and reuse of the same, green design (or low impact design) to reduce stormwater runoff, and use of greywater. The USEPA has developed several tools in the last ten years to address stormwater management under changing climate and they include:

  • • National Stormwater Calculator (SWC). This tool can provide estimates of runoff considering the historical and future climate.
  • - www.epa.gov/water-research/national-stormwater-calculator
  • • Storm Water Management Model and Climate Adjustment Tool (SWMM-CAT). This tool can be used to process regional downscaled climate projections and incorporate them into the Storm Water Management Model (SWMM) of the USEPA. This tool provides a set of monthly adjustment factors for time series that represent future climate change.
  • - www.epa.gov/water-research/storm-water-management-model-swmm
  • • Climate Resilience Evaluation and Awareness Tool (CREAT). This tool is mainly created for helping water utility owners and operators to understand climate change and its implications on the operation and management of these utilities.
  • - www.epa.gov/crwu/creat-risk-assessment-application-water-utilities

The U.S. Army Corps of Engineers (USAGE) has developed a guidance document (referred to as the Engineer Technical Letter (ETL) 1100-2-3) and a tool to detect nonstationarities in observed annual maximum discharges to support its project planning, design, and operations (USAGE, 2017). According to the USACE, the ETL (ETL 1100-2-3), engineers will be required to assess the stationarity of all streamflow records analyzed in support of hydrologic analysis carried out for USACE planning and engineering decision-making purposes. The nonstationarity detection tool developed by the USACE uses multiple statistical methods to detect the presence of both abrupt and smooth nonstationarities in the period of record. The tool also provides multiple trend analysis tests to evaluate changes in time series. The web-based tool with a nonstationarity detector and trend analysis is available at:

- http://corpsmapu.usace.army.mil/cm_apex/f?p=257:2:0::NO

Recommendations for Climate Change-Sensitive Hydrologic Design

In many regions of the U.S. and across the world, efforts are underway to understand

the changing characteristics of precipitation over time due to climate change and variability. The following recommendations are made for climate-change and variability

informed sustainable hydrologic design.

  • • A comprehensive evaluation of historical and climate-change model projection-based precipitation extremes is needed as a first step. Historical precipitation data need to be checked for any issues (viz., duplicate records, missing data, homogeneity issues related to station/site relocation, or instrumentation changes).
  • • Future projections of precipitation magnitudes will require the use of available spatially and temporally downscaled general circulation model-based outputs considering different scenarios. However, assessment of the suitability of one or more models to a specific region needs to be conducted. Furthermore, before the outputs (i.e., future precipitation magnitudes) can be used, an exhaustive evaluation of different GCM models needs to be conducted for their capabilities in replicating the characteristics of historical extremes.
  • • Climate change-based projections of precipitation at a local or regional scale depend on the use of robust spatial downscaling methods. An appropriate statistical and dynamic downscaling model for a specific region needs to be selected by using several performance metrics and its ability to replicate (i) spatial and temporal variability of precipitation in the region and (ii) oscillatory changes resulting from natural variability.
  • • Although daily precipitation extremes for specific return periods are often used for hydrologic infrastructure design, sub-daily precipitation values are also critical for many applications. Disaggregation models are required to obtain precipitation extremes at a finer temporal resolution based on coarse-resolution future projections. Appropriate disaggregation approaches need to be selected by exhaustive evaluation of their capabilities in resolving precipitation at finer temporal resolution with the help of historical data.
  • • Regional assessments of recent changes in precipitation extremes based not only on data from rain gauge observations but also on weather radar and satellite-based quantitative precipitation estimates (QPEs) are needed.
  • • Use of weather radar is recommended to assess and confirm the existence of rare precipitation extremes that are not observed by rain gauges due to several reasons (viz., sparse rain gauge networks, lack of rain gauges in a region, malfunctioning of rain gauges, and the spatial orientation of storms). Weather radar-based QPEs can also help in probable maximum precipitation (PMP) estimates.
  • • Periodical updates to available precipitation extreme databases and revisions to IDF relationships are required to support hydrologic design by incorporating changes occurring in evolving extremes. While updates are taking place, the upper limit of the 90% confidence interval estimates of precipitation magnitudes provided by the NOAA can be used to develop conservative hydrologic designs.
  • • The development of new approaches (e.g., Bayesian inference approach) that can consider nonstationarity of the precipitation extremes and adoption of these approaches by regional water management agencies is recommended. Aggregation techniques that pool several observations in a region are recommended based on the success of a recent study. It is also important that these emerging approaches should be evaluated on a case-by-case basis and successful applications should transform into new procedures for design.
  • • In many situations, only regional IDF relationships are available and they do not provide adequate information about changing precipitation extremes at the sub-regional scale. Therefore, in many instances, local IDF relationships need to be developed based on available rain gauge observations and QPEs from other estimation sources.
  • • Rainfall extremes are not the only drivers of floods. Recent research studies have focused on the development of IDF curves which consider both snowmelt processes and climate nonstationarity. These curves are referred to as next-generation IDF curves in multiple research studies, and these focus on the water available for runoff generation. More research studies are needed to understand snowmelt runoff generation mechanisms and rain-on-snow conditions that lead to catastrophic floods.
  • • The future hydrologic design should also consider approaches that use the concept of inter-event times via an inter-event time definition (IETD) that can identify extreme runoff generation scenarios for better design of hydrologic and hydraulic infrastructure.
  • • Long-duration precipitation extremes with different return periods need to be estimated considering the storm events that last over a day in many regions in the world including the U.S., which experiences events that are cyclonic with slow-moving hurricanes over the land (cyclones or typhoons).
  • • Hydrologic design procedures not only require extreme precipitation depths for the pre-specified return period but also need the intra-storm temporal distribution of precipitation when hydrologic simulation models are used. In a changing climate, the standard synthetic distributions (e.g., in the U.S. Soil Conservation Service (SCS) or Natural Resources Conservation Service (NRCS) synthetic rainfall distributions) are no longer valid. Therefore, there is a need for developing region-specific temporal distributions of extreme storm events based on local precipitation data.
  • • In a warming climate, a strong association between temperature and precipitation extremes is noted. This relationship, when quantified regionally, will be beneficial in specifying the spatial and temporal variation of precipitation extremes in hydrologic simulation models used for design.
  • • Optimization models that benefit from the soft computing paradigms can be used to develop compromise sustainable climate change-sensitive hydrologic design approaches. These approaches can model the uncertainties associated with multi-model multiple-scenario-based future climate projections.
  • • Updated PMP estimates in many regions of the U.S. are not available due to several reasons, which include funds to support studies.
  • • Atmospheric rivers (ARs) are known to be responsible for the occurrences of rare precipitation extremes in many parts of the U.S. in the past decade. A clear understanding of these systems and modeling approaches for evaluating these systems are required to forecast events in the short-term, and historical observations of extremes caused by such processes should be considered for PFA.
  • • Close collaboration among climate scientists, practicing engineers, hydrologists, and agencies that oversee and regulate hydrologic design is needed to work on addressing climate change in design standards. The development of guidelines to consider climate change in all aspects of design to reduce vulnerabilities and improve the resilience of existing infrastructure is critical.
  • • Climate change impacts on urban drainage systems can be reduced by using green infrastructure (GI) for both mitigation and adaptation efforts. Approaches that consider incorporation of GI for stormwater management, performance evaluation of existing urban drain systems, and adoption of emerging technologies to assist low impact development (LID) are required.
 
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