HYDROGEOLOGIC EVALUATION OF WETLAND BASINS FOR LAND USE PLANNING1

ABSTRACT: Wetlands occur in a variety of geologic, hydrologic, and topographic settings and exhibit diverse hydrogeologic characteristics. A wetland is more than an organic mat - it is an element in a larger hydrogeologic system. Three distinct but related effects of wetlands are: modifying the character of runoff from a basin, influencing the discharge/recharge relationship with the underlying aquifer, and effecting the potential for ground water development in a wetland dominated basin. An important goal of wetland research is to define the diverse roles that wetlands play in the regional hydrology and to define the geologic, hydrologic, and topographic factors that will allow meaningful distinctions among wetlands. Geologic and hydrologic factors include character and thickness of surficial materials; bedrock type; hydrologic position; permeability of organic layer; transmissivity, quality, and hydrologic connection of wetland related aquifers; ground water outflow; and depth of standing water. Topographic factors are position in the drainage basin, relative size, and absolute size of wetlands. A wetland classification to aid hydrologists and land use planners is proposed using selected factors involving hydrologic position, topographic position, and geologic type.     

Does Wetland Location Matter When Managing Wetlands for Watershed‐Scale Flood and Drought Resilience?

Wetland protection and restoration strategies that are designed to promote hydrologic resilience do not incorporate the location of wetlands relative to the main stream network. This is primarily attributed to the lack of knowledge on the effects of wetland location on wetland hydrologic function (e.g., flood and drought mitigation). Here, we combined a watershed‐scale, surface–subsurface, fully distributed, physically based hydrologic model with historical, existing, and lost (drained) wetland maps in the Nose Creek watershed in the Prairie Pothole Region of North America to (1) estimate the hydrologic functions of lost wetlands and (2) estimate the hydrologic functions of wetlands located at different distances from the main stream network. Modeling results showed wetland loss altered streamflow, decreasing baseflow and increasing stream peakflow during the period of the precipitation events that led to major flooding in the watershed and downstream cities. In addition, we found that wetlands closer to the main stream network played a disproportionately important role in attenuating peakflow, while wetland location was not important for regulating baseflow. The findings of this study provide information for watershed managers that can help to prioritize wetland restoration efforts for flood or drought risk mitigation.     

Modeling the Effects of Tile Drain Placement on the Hydrologic Function of Farmed Prairie Wetlands

The early 2000s saw large increases in agricultural tile drainage in the eastern Dakotas of North America. Agricultural practices that drain wetlands directly are sometimes limited by wetland protection programs. Little is known about the impacts of tile drainage beyond the delineated boundaries of wetlands in upland catchments that may be in agricultural production. A series of experiments were conducted using the well‐published model WETLANDSCAPE that revealed the potential for wetlands to have significantly shortened surface water inundation periods and lower mean depths when tile is placed in certain locations beyond the wetland boundary. Under the soil conditions found in agricultural areas of South Dakota in North America, wetland hydroperiod was found to be more sensitive to the depth that drain tile is installed relative to the bottom of the wetland basin than to distance‐based setbacks. Because tile drainage can change the hydrologic conditions of wetlands, even when deployed in upland catchments, tile drainage plans should be evaluated more closely for the potential impacts they might have on the ecological services that these wetlands currently provide. Future research should investigate further how drainage impacts are affected by climate variability and change.     

Strategies for assessing the cumulative effects of wetland alteration on water quality

Assessment of cumulative impacts on wetlands can benefit by recognizing three fundamental wetland categories: basin, riverine, and fringe. The geomorphological settings of these categories have relevance for water quality.Basin, or depressional, wetlands are located in headwater areas, and capture runoff from small areas. Thus, they are normally sources of water with low elemental concentration. Although basin wetlands normally possess a high capacity for assimilating nutrients, there may be little opportunity for this to happen if the catchment area is small and little water flows through them.Riverine wetlands, in contrast, interface extensively with uplands. It has been demonstrated that both the capacity and the opportunity for altering water quality are high in riverine wetlands.Fringe wetlands are very small in comparison with the large bodies of water that flush them. Biogeochemical influences tend to be local, rather than having a measurable effect on the larger body of water. Consequently, the function of these wetlands for critical habitat may warrant protection from high nutrient levels and toxins, rather than expecting them to assume an assimilatory role.The relative proportion of these wetland types within a watershed, and their status relative to past impacts can be used to develop strategies for wetland protection. Past impacts on wetlands, however, are not likely to be clearly revealed in water quality records from monitoring studies, either because records are too short or because too many variables other than wetland impacts affect water quality. It is suggested that hydrologic records be used to reconstruct historical hydroperiods in wetlands for comparison with current, altered conditions. Changes in hydroperiod imply changes in wetland function, especially for biogeochemical processes in sediments. Hydroperiod is potentially a more sensitive index of wetland function than surface areas obtained from aerial photographs. Identification of forested wetlands through photointerpretation relies on vegetation that may remain intact for decades after drainage. Finally, the depositional environment of wetlands is a landscape characteristic that has not been carefully evaluated nor fully appreciated. Impacts that reverse depositional tendencies also may accelerate rates of change, causing wetlands to be large net exporters rather than modest net importers. Increases in rates as well as direction can cause stocks of materials, accumulated over centuries in wetland sediments, to be lost within decades, resulting in nutrient loading to downstream aquatic ecosystems.     

Evaluating cumulative effects of disturbance on the hydrologic function of bogs,fens, and mires

Few quantitative studies have been done on the hydrology of fens, bogs, and mires. Consequently predicting the cumulative impacts of disturbances on their hydrologic functions is extremely difficult. For example, few data are available on the role of bogs and fens with respect to flood desynchronization and shoreline anchoring. However, recent studies suggest that very small amounts of groundwater discharge are sufficient to radically modify mire surface-water chemistry, and consequently, vegetation communities and their associated surface-water hydrology. Bogs and fens are, in a sense, hydrobiologic systems, and any evaluation of cumulative impacts will have to (1) consider the complicated and little understood interactions among wetland hydrology, water chemistry, and biota, and (2) place the effect of individual wetland impacts within the context of the cumulative impacts contributed to the watershed from other geomorphic areas and land uses.It is difficult to evaluate the potential cumulative impacts on wetland hydrology because geologic settings of wetlands are often complex and the methods used to measure wetland streamflow, groundwater flow, and evapotranspiration are inexact (Winter 1988). This is especially so for bogs, fens, and mires underlain by thick organic soils. These wetlands, found in the circumboreal areas of North America, Europe, and Asia, are major physiographic features in eastern North America, northern Europe, and Siberia (Kivenen and Pakarinen 1981, Gore 1983, Glaser and Janssens 1986). Their very scale makes it difficult to quantify the hydrologic function accurately. The hydrology of small bogs and fens found elsewhere is just as poorly understood because of conflicting conceptual models of pertinent hydrologic processes.This article (1) reviews our current understanding of the hydrologic function of bogs, fens, and mires at different scales and in different physiographic settings and (2) presents hypotheses on potential cumulative impacts on the hydrologic function that might occur with multiple disturbances.     

THE VULNERABILITY OF WETLANDS TO CLIMATE CHANGE: A HYDROLOGIC LANDSCAPE PERSPECTIVE1

ABSTRACT: The vulnerability of wetlands to changes in climate depends on their position within hydrologic landscapes. Hydrologic landscapes are defined by the flow characteristics of ground water and surface water and by the interaction of atmospheric water, surface water, and ground water for any given locality or region. Six general hydrologic landscapes are defined; mountainous, plateau and high plain, broad basins of interior drainage, riverine, flat coastal, and hummocky glacial and dune. Assessment of these landscapes indicate that the vulnerability of all wetlands to climate change fall between two extremes: those dependent primarily on precipitation for their water supply are highly vulnerable, and those dependent primarily on discharge from regional ground water flow systems are the least vulnerable, because of the great buffering capacity of large ground water flow systems to climate change.     

Simulated wetland conservation-restoration effects on water quantity and quality at watershed scale

Wetlands are one of the most important watershed microtopographic features that affect hydrologic processes (e.g., routing) and the fate and transport of constituents (e.g., sediment and nutrients). Efforts to conserve existing wetlands and/or to restore lost wetlands require that watershed-level effects of wetlands on water quantity and water quality be quantified. Because monitoring approaches are usually cost or logistics prohibitive at watershed scale, distributed watershed models such as the Soil and Water Assessment Tool (SWAT), enhanced by the hydrologic equivalent wetland (HEW) concept developed by Wang [Wang, X., Yang, W., Melesse, A.M., 2008. Using hydrologic equivalent wetland concept within SWAT to estimate streamflow in watersheds with numerous wetlands. Trans. ASABE 51 (1), 55–72.], can be a best resort. However, there is a serious lack of information about simulated effects using this kind of integrated modeling approach. The objective of this study was to use the HEW concept in SWAT to assess effects of wetland restoration within the Broughton's Creek watershed located in southwestern Manitoba, and of wetland conservation within the upper portion of the Otter Tail River watershed located in northwestern Minnesota. The results indicated that the HEW concept allows the nonlinear functional relations between watershed processes and wetland characteristics (e.g., size and morphology) to be accurately represented in the models. The loss of the first 10–20% of the wetlands in the Minnesota study area would drastically increase the peak discharge and loadings of sediment, total phosphorus (TP), and total nitrogen (TN). On the other hand, the justifiable reductions of the peak discharge and loadings of sediment, TP, and TN in the Manitoba study area may require that 50–80% of the lost wetlands be restored. Further, the comparison between the predicted restoration and conservation effects revealed that wetland conservation seems to deserve a higher priority while both wetland conservation and restoration may be equally important.     

A conceptual framework for assessing cumulative impacts on the hydrology of nontidal wetlands

Wetlands occur in geologic and hydrologic settings that enhance the accumulation or retention of water. Regional slope, local relief, and permeability of the land surface are major controls on the formation of wetlands by surface-water sources. However, these landscape features also have significant control over groundwater flow systems, which commonly play a role in the formation of wetlands. Because the hydrologic system is a continuum, any modification of one component will have an effect on contiguous components. Disturbances commonly affecting the hydrologic system as it relates to wetlands include weather modification, alteration of plant communities, storage of surface water, road construction, drainage of surface water and soil water, alteration of groundwater recharge and discharge areas, and pumping of groundwater. Assessments of the cumulative effects of one or more of these disturbances on the hydrologic system as related to wetlands must take into account uncertainty in the measurements and in the assumptions that are made in hydrologic studies. For example, it may be appropriate to assume that regional groundwater flow systems are recharged in uplands and discharged in lowlands. However, a similar assumption commonly does not apply on a local scale, because of the spatial and temporal dynamics of groundwater recharge. Lack of appreciation of such hydrologic factors can lead to misunderstanding of the hydrologic function of wetlands within various parts of the landscape and mismanagement of wetland ecosystems.     

Hydrologic Landscape Classification to Estimate Bristol Bay,Alaska Watershed Hydrology

Hydrologic landscapes (HLs) have proven to be a useful tool for broad scale assessment and classification of landscapes across the United States as they help organize larger geographical areas into areas of similar hydrologic characteristics. We developed a HL classification for the Bristol Bay watershed of southwest Alaska that incorporates indices of annual climate and seasonality, terrain, geology, and the influences of large lakes and glaciers. A HL classification is particularly useful in this large watershed because of its hydrologic and landscape variability, important salmon fishery, variety of environmental and potential anthropogenic stressors, and lack of widespread hydrologic data. Following creation of Bristol Bay basin‐wide HL classes, we compared the HL distributions within watersheds grouped by two calculated runoff parameters derived from available long‐term streamflow records and found HL distributions within these groups provided predictive insight on hydrologic behavior. Using these developed runoff groups, we estimated expected hydrologic behavior in watersheds across the larger Bristol Bay watershed that lacked gauged streamflow records. The HL approach provides a scientific basis for estimating the first‐order hydrologic behavior of watersheds and landscapes that lack detailed hydrologic information.     

Reducing Streamflow Forecast Uncertainty: Application and Qualitative Assessment of the Upper Klamath River Basin,Oregon1

Abstract: The accuracy of streamflow forecasts depends on the uncertainty associated with future weather and the accuracy of the hydrologic model that is used to produce the forecasts. We present a method for streamflow forecasting where hydrologic model parameters are selected based on the climate state. Parameter sets for a hydrologic model are conditioned on an atmospheric pressure index defined using mean November through February (NDJF) 700‐hectoPascal geopotential heights over northwestern North America [Pressure Index from Geopotential heights (PIG)]. The hydrologic model is applied in the Sprague River basin (SRB), a snowmelt‐dominated basin located in the Upper Klamath basin in Oregon. In the SRB, the majority of streamflow occurs during March through May (MAM). Water years (WYs) 1980‐2004 were divided into three groups based on their respective PIG values (high, medium, and low PIG). Low (high) PIG years tend to have higher (lower) than average MAM streamflow. Four parameter sets were calibrated for the SRB, each using a different set of WYs. The initial set used WYs 1995‐2004 and the remaining three used WYs defined as high‐, medium‐, and low‐PIG years. Two sets of March, April, and May streamflow volume forecasts were made using Ensemble Streamflow Prediction (ESP). The first set of ESP simulations used the initial parameter set. Because the PIG is defined using NDJF pressure heights, forecasts starting in March can be made using the PIG parameter set that corresponds with the year being forecasted. The second set of ESP simulations used the parameter set associated with the given PIG year. Comparison of the ESP sets indicates that more accuracy and less variability in volume forecasts may be possible when the ESP is conditioned using the PIG. This is especially true during the high‐PIG years (low‐flow years).     

Predicting streamflow response to fire-induced landcover change: implications of parameter uncertainty in the MIKE SHE model

Fire is a primary agent of landcover transformation in California semi-arid shrubland watersheds, however few studies have examined the impacts of fire and post-fire succession on streamflow dynamics in these basins. While it may seem intuitive that larger fires will have a greater impact on streamflow response than smaller fires in these watersheds, the nature of these relationships has not been determined. The effects of fire size on seasonal and annual streamflow responses were investigated for a medium-sized basin in central California using a modified version of the MIKE SHE model which had been previously calibrated and tested for this watershed using the Generalized Likelihood Uncertainty Estimation methodology. Model simulations were made for two contrasting periods, wet and dry, in order to assess whether fire size effects varied with weather regime. Results indicated that seasonal and annual streamflow response increased nearly linearly with fire size in a given year under both regimes. Annual flow response was generally higher in wetter years for both weather regimes, however a clear trend was confounded by the effect of stand age. These results expand our understanding of the effects of fire size on hydrologic response in chaparral watersheds, but it is important to note that the majority of model predictions were largely indistinguishable from the predictive uncertainty associated with the calibrated model - a key finding that highlights the importance of analyzing hydrologic predictions for altered landcover conditions in the context of model uncertainty. Future work is needed to examine how alternative decisions (e.g., different likelihood measures) may influence GLUE-based MIKE SHE streamflow predictions following different size fires, and how the effect of fire size on streamflow varies with other factors such as fire location.     

Groundwaters at Risk: Wetland Loss Changes Sources,Lengthens Pathways,and Decelerates Rejuvenation of Groundwater Resources

Wetland loss alters the hydrology of wetlandscapes in poorly understood ways. To quantify the effects of wetland loss on subsurface hydrology, a physically based hydrologic model that simulates the timing and pathways of subsurface hydrologic connections was coupled with wetland inventories over a 50‐year period during which substantial wetland loss occurred. The model revealed, based on vertical variations in saturated hydraulic conductivities, wetland loss of different degrees led to a contraction of catchment contributing areas to local surface waters but an expansion of contributing areas to the regional surface water body. This shift in groundwater contributing areas reflected (1) a decrease in baseflow contribution to the local surface water bodies, and (2) an increase in the transit time and length of subsurface hydrologic connections with an associated increase in the magnitude and age of baseflow discharging to the regional surface water body. The model also showed regions with thick permeable aquifers were particularly sensitive to the loss of wetlands. Our ability to predict these changes in hydrology of the watershed provides important support for designing science‐based policies to promote sustainable water resource management.     

Capturing LiDAR‐Derived Hydrologic Spatial Parameters to Evaluate Playa Wetlands

The digital elevation model data from traditional stereo photogrammetric methods are inadequate in providing accurate vertical parameters to feed hydrologic models for low‐lying, extremely flat areas. High‐resolution light detection and ranging (LiDAR) data provide the robust capability of capturing small variations in low‐relief playa wetlands. The Rainwater Basin in south‐central Nebraska includes a complex of seasonally shallow playa wetlands that attract millions of migratory waterfowl every spring and fall. This research focuses on the development of a procedure with applicable protocols to produce LiDAR‐derived three‐dimensional wetland maps and to extract the critical surface parameters (i.e., watershed boundaries, flow direction, flow accumulation, and drainage lines) for playa wetlands. The topo‐hydrologic conditions of playa wetlands were evaluated at the watershed level. The results show that in the Rainwater Basin, 70.7% of the historic hydric soil footprints identified in the Soil Survey Geographic (SSURGO) database were not functioning as topographically depressional wetlands. This finding was confirmed by a recent five‐year Annual Habit Survey showing that 69.8% of the historic hydric soil footprints did not function during the spring migratory bird seasons between 2004 and 2009. The majority of playa wetlands' topographic conditions have been substantially changed and the SSURGO data cannot fully reflect current topographic reality in the Rainwater Basin.     

Physical and Temporal Isolation of Mountain Headwater Streams in the Western Mojave Desert,Southern California1

Abstract: Streams draining mountain headwater areas of the western Mojave Desert are commonly physically isolated from downstream hydrologic systems such as springs, playa lakes, wetlands, or larger streams and rivers by stream reaches that are dry much of the time. The physical isolation of surface flow in these streams may be broken for brief periods after rainfall or snowmelt when runoff is sufficient to allow flow along the entire stream reach. Despite the physical isolation of surface flow in these streams, they are an integral part of the hydrologic cycle. Water infiltrated from headwater streams moves through the unsaturated zone to recharge the underlying ground‐water system and eventually discharges to support springs, streamflow, isolated wetlands, or native vegetation. Water movement through thick unsaturated zones may require several hundred years and subsequent movement through the underlying ground‐water systems may require many thousands of years – contributing to the temporal isolation of mountain headwater streams.     

Land loss in coastal Louisiana (U.S.A.)

This paper examines causes and consequences of wetland losses in coastal Louisiana. Land loss is a cumulative impact, the result of many impacts both natural and artificial. Natural losses are caused by subsidence, decay of abandoned river deltas, waves, and storms. Artificial losses result from flood-control practices, impoundments, and dredging and subsequent erosion of artificial channels. Wetland loss also results from spoil disposal upon wetlands and land reclamation projects.Total land loss in Louisiana's coastal zone is at least 4,300 ha/year. Some wetlands are converted to spoil banks and other eco-systems so that wetland losses are probably two to three times higher. Annual wetland losses in the Barataria Bay basin are 2.6% of the wetland area. Human activities are the principal determinants of land loss. The present total wetland area directly lost because of canals may be close to 10% if spoil area is included. The interrelationship between hydrology, land, vegetation, substrate, subsidence, and sediment supply are complicated; however, hydrologic units with high canal density are generally associated with higher rates of land loss and the rate may be accelerating.Some cumulative impacts of land loss are increased saltwater intrusion, loss of capacity to buffer the impact of storms, and large additions of nutrients. One measure of the impact is that roughly $8–17 × 10⁶ (U.S.A.) of fisheries products and services are lost annually in Louisiana.Viewed at the level of the hydrologic unit, land loss transcends differences in local vegetation, substrate, geology, and hydrology. Land management should therefore focus at that level of organization. Proper guideline recommendations require an appreciation of the long-term interrelations of the wetland estuarine system.     

Spatial Relationships of Levees and Wetland Systems within Floodplains of the Wabash Basin,USA

Given the unique biogeochemical, physical, and hydrologic services provided by floodplain wetlands, proper management of river systems should include an understanding of how floodplain modifications influence wetland ecosystems. The construction of levees can reduce river–floodplain connectivity, yet it is unclear how levees affect wetlands within floodplains, let alone the cumulative impacts within an entire watershed. This paper explores spatial relationships between levee and floodplain wetland systems in the Wabash Basin, United States. We used a hydrogeomorphic floodplain delineation technique to map floodplain extents and identify wetlands that may be hydrologically connected to river networks. We then spatially examined the relationship between levee presence, wetland area, and other river network attributes within discrete subbasins. Our results show that cumulative wetland area is relatively constant in subbasins that contain levees, regardless of maximum stream order within the subbasin. In subbasins that do not contain levees, cumulative wetland area increases with maximum stream order. However, we found that wetland distributions around levees can be complex, and further studies on the influence of levees on wetland habitat may need to consider finer resolution spatial scales.     

A rationale for coastal wetland restoration through spoil bank management in Louisiana,USA

The rationale and outline of an implementation plan for restoring coastal wetlands in Louisiana is presented. The rationale for the plan is based on reversing the consequences of documented cause-and-effect relationships between wetland loss and hydrologic change. The main feature is to modify the extensive interlocking network of dredged spoil deposits, or spoil banks, by reestablishing a more natural water flow at moderate flow velocity (<5 cm/sec). Guidelines for site selection from thousands of potential sites are proposed. Examples of suitable sites are given for intermediate marshes. These sites exhibit rapid deterioration following partial or complete hydrologic impoundment, implying a strong hydrologic, rather than sedimentological, cause of wetland deterioration. We used an exploratory hydrologic model to guide determination of the amount of spoil bank to be removed. The results from an economic model indicated a very effective cost-benefit ratio. Both models and practical experience with other types of restoration plans, in Louisiana and elsewhere, exhibit an economy of scale, wherein larger projects are more cost effective than smaller projects. However, in contrast to these other projects, spoil bank management may be 100 to 1000 times more cost effective and useful in wetland tracts <1000 ha in size. Modest spoil bank management at numerous small wetland sites appears to offer substantial positive attributes compared to alternative and more intensive management at a few larger wetland sites.     

Cumulative impacts on water quality functions of wetlands

The total effect of cumulative impacts on the water quality functions of wetlands cannot be predicted from the sum of the effects each individual impact would have by itself. The wetland is not a simple filter; it embodies chemical, physical, and biotic processes that can detain, transform, release, or produce a wide variety of substances. Because wetland water quality functions result from the operation of many individual, distinct, and quite dissimilar mechanisms, it is necessary to consider the nature of each individual process.Sound knowledge of the various wetland processes is needed to make guided judgements about the probable effects of a given suite of impacts. Consideration of these processes suggests that many common wetland alterations probably do entail cumulative impact. In addition to traditional assessment methods, the wetland manager may need to obtain appropriate field measurements of water quality-related parameters at specific sites; such data can aid in predicting the effects of cumulative impact or assessing the results of past wetland management.     

Review and comparison of wetland impacts and mitigation requirements between New Jersey,USA, Freshwater Wetlands Protection Act and Section 404 of the Clean Water Act

A review of wetland impacts authorized under the New Jersey Freshwater Wetlands Protection Act (FWPA) was conducted based on permitting data compiled for the period 1 July 1988 to 31 December 1993. Data regarding the acreage of wetlands impacted, location of impacts by drainage basin and watershed, and mitigation were analyzed. Wetland impacts authorized and mitigation under New Jersey's program were evaluated and compared with Section 404 information available for New Jersey and other regions of the United States.Under the FWPA, 3003 permits were issued authorizing impacts to 234.76 ha (602.27 acres) of wetlands and waters. Compensatory mitigation requirements for impacts associated with individual permits required the creation of 69.20 ha. (171.00 acres), and restoration of 16.49 ha (40.75 acres) of wetlands. Cumulative impacts by watershed were directly related to levels of development and population growth.The FWPA has resulted in an estimated 67% reduction [44.32 ha (109.47 acres) vs 136.26 ha (336.56 acres)] in annual wetland and water impacts when compared with Section 404 data for New Jersey. For mitigation, the slight increase in wetland acreage over acreage impacted is largely consistent with Section 404 data.Based on this evaluation, the FWPA has succeeded in reducing the level of wetland impacts in New Jersey. However, despite stringent regulation of activities in and around wetlands, New Jersey continues to experience approximately 32 ha (79 acres) of unmitigated wetland impacts annually. Our results suggest that additional efforts focusing on minimizing wetland impacts and increasing wetlands creation are needed to attain a goal of no net loss of freshwater wetlands.     

Mid‐Range Streamflow Forecasts Based on Climate Modeling – Statistical Correction and Evaluation1

Abstract: Mid‐range streamflow predictions are extremely important for managing water resources. The ability to provide mid‐range (three to six months) streamflow forecasts enables considerable improvements in water resources system operations. The skill and economic value of such forecasts are of great interest. In this research, output from a general circulation model (GCM) is used to generate hydrologic input for mid‐range streamflow forecasts. Statistical procedures including: (1) transformation, (2) correction, (3) observation of ensemble average, (4) improvement of forecast, and (5) forecast skill test are conducted to minimize the error associated with different spatial resolution between the large‐scale GCM and the finer‐scale hydrologic model and to improve forecast skills. The accuracy of a streamflow forecast generated using a hydrologic model forced with GCM output for the basin was evaluated by forecast skill scores associated with the set of streamflow forecast values in a categorical forecast. Despite the generally low forecast skill score exhibited by the climate forecasting approach, precipitation forecast skill clearly improves when a conditional forecast is performed during the East Asia summer monsoon, June through August.