Recession analysis revisited: impacts of climate on parameter estimation

Recession analysis is a classical method in hydrology to assess watersheds' hydrological properties by means of the receding limb of a hydrograph, frequently expressed as the rate of change in discharge (-dQ/dt) against discharge (Q). This relationship is often assumed to take the form of a power law -dQ/dt=aQb, where a and b are recession parameters. Recent studies have highlighted major differences in the estimation of the recession parameters depending on the method, casting doubt on our ability to properly evaluate and compare hydrological properties across watersheds based on recession analysis of -dQ/dt vs. Q. This study shows that estimation based on collective recessions as an average watershed response is strongly affected by the distributions of event inter-arrival time, magnitudes, and antecedent conditions, implying that the resulting recession parameters do not represent watershed properties as much as they represent the climate. The main outcome from this work highlights that proper evaluation of watershed properties is only ensured by considering independent individual recession events. While average properties can be assessed by considering the average (or median) values of a and b, their variabilities provide critical insight into the sensitivity of a watershed to the initial conditions involved prior to each recharge event.

The Effect of QPF on Real-Time Deterministic Hydrologic Forecast Uncertainty

The use of quantitative precipitation forecast (QPF) in hydrologic forecasting is commonplace, but QPF is subject to considerable error. When QPF is included as a model forcing in the hydrological forecast process, significant error propagates through the hydrologic predictions. Two questions arise: 1) are the resulting observed hydrologic forecast errors sufficiently large to suggest the use of zero QPF in the forecast process, and 2) if the use of nonzero QPF is indicated, how many periods (hours) of QPF (1, 6, 12, …, 72 h) should be used? Also, do forecast conditions exist under which the use of QPF should be different? This study presents results from two real-time hydrologic forecast experiments, focused on the NOAA/NWS Ohio River Forecast Center (OHRFC). The experiments rely on forecasts from subbasins at 39 forecast point locations, ranging in drainage area, geographic location within the Ohio River Valley, and watershed response time. Results from an experiment, spanning all flow ranges, for the 10 August 2007–31 August 2009 period, show that nonzero QPF produces smaller hydrologic forecast error than zero QPF. A second experiment, 23 January 2009–15 September 2010, suggests that QPF should be limited to 6–12-h duration for flood forecasts. Beyond 12 h, hydrologic forecast error increases substantially across all forecast ranges, but errors are much larger for flood forecasts. Increased durations of QPF produce smaller forecast error than shorter QPF durations only for nonflood forecasts. Experimental results are shown to be consistent with NWS April 2001–October 2016 forecast verification statistics for the OHRFC.

Recession analysis 42 years later – work yet to be done

Recession analysis is a classical method employed in hydrology to assess watersheds’ hydrological properties by means of the receding limb of a hydrograph, frequently expressed as the rate of change in discharge (dQ/dt) against discharge (Q). This relationship is often assumed to take the form of a power law −dQ/dt = aQb where a and b are recession parameters. Recent studies have highlighted major differences in the estimation of the recession parameters depending on the method, casting doubt on our ability to properly evaluate and compare hydrological properties across watersheds based on recession analysis. This study shows that estimation based on collective recessions as an average watershed response is strongly affected by the distributions event inter-arrival time, magnitudes, and antecedent conditions, implying that the resulting recession parameters do not represent watershed properties as much as they represent the climate. The clear conclusion is that proper evaluation of watershed properties using recession analysis requires considering individual recession events.

Identification of Hydrologically Sensitive Areas Considering Watershed Process Dynamics

Hydrologically sensitive areas (HSAs) largely control watershed response to rainfall, along with water and pollutant transport processes. Thus, their identification is critical in watershed management planning. Although watershed processes have been studied enough to provide a good understanding of HSA dynamics, only a few concepts and methods are available for HSA delineation, and they rely on spatial indices that do not consider temporal variation in hydrologic processes. This study introduces alternative concepts and methods to delineate HSAs. Three unique maps showing watershed dynamics were created using the outputs of a long-term hydrologic simulation implemented with a grid-based distributed model. The spatial distributions of HSAs identified using the newly proposed methods were compared with those of topographic indices. Results demonstrated that the new methods highlight transport processes, such as routing (or connection) and travel time, and the roles of soil and land covers, which have not been the focus of other concepts and approaches for HSA identification. In contrast to topographic index-based approaches, the proposed methods provided HSA boundaries with clear physical meanings to improve the interpretability and applicability of HSA maps. The methods are expected to enhance our ability to tackle water issues for improved water resource management by providing unique concepts and alternative ways to explicitly delineate HSAs. Keywords: Grid-based distributed model, Hydrologic connectivity, Hydrologically sensitive area, HYSTAR, Time-area method, Topographic wetness index.

RETENTION CAPACITY CHANGES IN OPAK SUB WATERSHED POST-MERAPI VOLCANO ERUPTION 2010

Merapi Volcano Eruption in 2010 led to physical changes in the Opak Sub Watershed in Sleman Regency. They include changes in land use/cover and soil conditions which consequently change the watershed response to rainfall, as well as the nature of generated flood. This research aimed to (1) determine land use/cover change in the upstream area of Opak Watershed after Merapi Volcano eruption in 2010, and (2) assess the changes in water retention capacity of the soil after the eruption of Merapi Volcano in 2010 and its impact on the environment. Land use change was analyzed with temporal remote sensing imagery with high resolution. Retention capacity was assessed using SCS-CN method. The results of both assessments were, then, used to formulate recommendations for management in Opak Sub Watershed. The analysis showed that Merapi Volcano eruption in 2010 caused 63% of land use/cover to become vacant and changes in surface material and watershed boundaries. In addition, it was determined that the post-eruption retention capacity had been increasing. This indicates that changes in material are likely to affect retention capacity much more than changes in land use/cover.