Applying the Basin Characterization Model Dataset to Conservation Planning and Management Issues

Excerpted from:
Weiss, S., A. Flint, L. Flint, H. Hamilton, M. Fernandez, and L. Micheli. 2013. High Resolution Climate-Hydrology Scenarios for San Francisco’s Bay Area. Page 57 pp. Technical Report, Dwight Center for Conservation Science, Santa Rosa, CA.

Back to The Basin Characterization Model Downscaled Climate and Hydrology Dataset

Applying the BCM to Management Issues

How does one apply these data sets to management? All the futures are “wrong” in some sense; projecting annual weather in detail is impossible, but trends averaged over 30-years and associated variability statistics are internally consistent. Picking the “most likely” future is a futile exercise; precipitation in particular is highly variable among models. But the following trends are robust:

1) Temperatures will warm and the question is how fast and how much (see Figures 2 and 4).
2) CWD increases across all futures, because increased PET in the dry season is acting on limited soil moisture storage, and any excesses in PPT will result in recharge or runoff in the wet season. Therefore, from the viewpoint of vegetation the landscape will become more effectively arid in all futures (see Figure 6).
3) Recharge and runoff are direct functions of PPT – high monthly PPT, especially in mid-winter, goes directly to runoff once maximal recharge rates are satisfied.
4) Interannual variability and stressful multi-year events (i.e. droughts) will still pose the fundamental challenge to managers, and are an intrinsic feature of climate futures. Extreme events are what drive ecosystem changes and stress infrastructure.

In the absence of a most likely future, the suggested approach given uncertainties is to be “scenario-neutral” (Brown and Wilby 2013, Prudhomme et al. 2010). This approach uses different climate futures to examine the limits of acceptable system performance, e.g. the ability of a water supply system to meet demand, or the ability of a conservation network to support some minimum amount of a key vegetation type or species. In this approach, the first step is to define a threshold of failure in system performance. In short, ask the question “What does it take to break the system?” Then multiple futures and time periods are applied to the system, and the thresholds of failure over various time periods are determined probabilistically. In this ensemble, 54 options exist (18 futures x 3 time periods). Then, management options can be examined to see if the threat of system failure can be ameliorated or tolerated.

This approach puts the onus on managers to define the particulars of their systems of interest. Because each watershed/preserve/landscape or area of interest is unique, there is really no set answer a priori. Probabilities of vegetation transition are particularly relevant for land managers, but short of that, changes in productivity (AET) or increased drought stress (CWD), provide some metrics of landscape stress. Changes in recharge and runoff, of course, are directly relevant to water supply and aquatic ecosystems.

Choice of Futures

Any combination of future and time period can be used for assessing the likelihood of exceeding key thresholds; think of the 18 futures and 3 time periods as a menu for exploration of system resiliency in the face of varying degrees of climate change. Given the wide variety of futures to choose from, where should an assessment start? Several recommendations have come out of investigations.

1) There is no evidence to date that the highest emissions scenarios (RCP85 and SRES-A2) will be avoided, so consideration of these extreme scenarios is necessary.
2) An easily understandable climate future is one where the Pacific storm track moves northward, leading to drier conditions overall with a compression of the rainy season. Such a future provides spatial analogs from more southern regions in California; i.e. the Central Coast and Southern California.
3) Examining a worse-case future provides a benchmark for system performance.
4) There is much experience in California with the GFDL/PCM A2/B1 futures which span a range of Warmer-Hotter and Drier-Wetter.
5) Of those 4 futures, it is recommended that the GFDL-A2 be the starting point for assessment.
6) Once GFDL-A2 is assessed, then the other three futures should be considered.
7) A mid-century time period (see below) provides enough time for substantial climate change, but is still within a reasonable planning horizon.

Uses of BCM Output for Conservation Planning

The advantage of using BCM output over raw climate inputs is that the resultant outputs are integrated water balance variables that are more directly relevant to ecosystems and water supply.
Several approaches for using BCM output for climate change assessment for conservation are discussed below. They generally fall into three categories; direct targeting of important hydrologic and climatic areas; post hoc evaluation of climatic resilience of areas chosen for current conservation values; and management considerations.

Direct Targeting Opportunities

1) Direct targeting of key hydrologic resources:
Areas of high recharge that are the result of high precipitation, shallow soils, and high bedrock permeability are a good example. These locations are stable on the landscape under climate change, even if the absolute magnitude of recharge changes. Recharge is precious in our Mediterranean climate because it provides baseflow for streams during the dry season, and many rural communities depend on local groundwater. Key recharge areas can be prioritized by Planning Watersheds with high value aquatic resources (i.e. foothill yellow-legged frog, steelhead, and native fishes). Protecting locally high recharge areas from development, even low density rural development, prevents or reduces recharge from occurring due to disruptions in surface hydrology by roads, houses, and other infrastructure.

Similar reasoning applies to high runoff areas above water supply reservoirs, with the emphasis on maintaining water quality (primarily sediment in upper watersheds). The current period (1981-2010) recharge and runoff maps are good first order spatial representations of these important resources.

2) Direct targeting of local climatic refugia:
The coolest, moistest locations within a given area provide potential local refugia for species. Intact alluvial flats with deep soils, riparian corridors, and north-facing slopes are particularly important. Narrow riparian areas and many north-facing slopes are below the 270-m scale of the BCM outputs. Riparian areas are by themselves critical conservation targets under numerous criteria. For climate change resiliency, riparian zones can be considered as linear features that intersect BCM grid cells, and a distance to riparian function can be generated. Sub-grid solar radiation (at 30 m) can be nested within the BCM grid, and diversity measures (range and minimum solar radiation calculated for each 270-m cell (or any arbitrary area). This further downscaling is an area of active research.

3) Direct targeting of high diversity areas:
Local climatic diversity at scales from ~500 meters to several kilometers allows species to redistribute on local scales well-within the dispersal range of many species. Neighborhood range of BCM output at varying scales, or within defined areas (i.e. contiguous protected lands), is a powerful measure of local resilience. Comparisons between the magnitude of spatial variability and projected changes in climate are the crux of a risk analysis at a landscape scale, and some examples are presented below.

4) Direct targeting of resistant areas where climate changes less:
Climate change is not necessarily uniform across the landscape. There is spatial variability in historic climate changes; mountains have behaved differently than valley bottoms and lower foothills across much of the Bay Area. Deeper soils behave very differently than do shallow soils in response to reductions in precipitation, and somewhat paradoxically show larger absolute increases in CWD and reductions in AET with lower precipitation. The Standardized Euclidean distance between historic and projected climates integrates multiple factors into the magnitude of climate change in each BCM cell, and is presented below. Some caution is recommended here, because downscaling methods may create spatial artifacts, or greatly mute spatial variability in rates of change.

Post Hoc Evaluation of Climatic Resilience
Rather than use the BCM outputs as direct targets, a second approach is to evaluate existing conservation lands, conservation priorities identified by other criteria, and conservation opportunities in terms of contributions to climate change resiliency. The key concept here is that an expansion of spatially contiguous climate space increases resilience. The basic procedure is:
1) Define the area of interest: The best example is contiguous existing conservation lands and any potential additions.

2) Evaluate resiliency metrics within the existing lands: Many of the analyses described above can be incorporated; perhaps the simplest is the range and proportional distribution of key factors (i.e. CWD) within the area.
3) Add in proposed new lands and re-evaluate resiliency metrics: Recalculate the range and proportional distribution of key factors with the additional lands and evaluate expansion of climate space within the contiguous area.
4) Connectivity effects: If the new lands make a connection between existing complexes of conservation lands, then evaluate the entire newly contiguous area.
A potential initial application of this procedure is to identify the highest conservation priorities based on the numerous other criteria (i.e. multiple benefit areas) and add them to the existing network, then evaluates the climatic range within the new conservation network pieces.
This analysis is also applicable to water resources. Additions to protected recharge areas within Planning Watersheds can be quantified by calculating the volume of recharge provided by those lands; similarly runoff into reservoir catchments can also be assessed.

Implications for Land Management

Changing hydrologic balance will manifest itself in the following ways that will pose management challenges.

1) Higher CWD can lead to direct mortality of existing vegetation through drought stress. Vegetation composition and structure will change – woodlands will thin, even to the point of loss of trees in open oak savannas that lead to conversion to grasslands. Drought stressed vegetation is also more susceptible to insect outbreaks and pathogens. Dead standing trees pose safety hazards especially in heavily used recreational areas.
2) Higher CWD increases fire risk and intensity. Fire seasons will lengthen, and drier vegetation is more flammable. Fire protection and management become ever more important. Post-fire weed management is critical if native vegetation is to occupy burnt areas. Post-fire erosion control is also critical for protection of water quality.
3) Decreased AET leads to reduced net primary productivity, with cascading effects up the food chain. Wildlife may suffer food shortages if key plant resources are less productive, and rangeland productivity greatly affects grazing management.
4) Increased AET leads to increased net primary productivity which may appear to be a good thing. However, combined with increased CWD increased productivity can lead to increased fuel loads and higher fire intensities. Fuels management, especially at the urban-wildland interface may become a higher priority where AET increases locally.
5) Decreased runoff will lead to lower chances of filling ponds, reservoirs, and intermittent streams during drier than average periods. Water for wildlife, both aquatic and terrestrial, may become highly restricted in low runoff areas. Lack of sufficient runoff pulses can hinder the movement of anadramous fish upstream, especially if streams have partial fish barriers.
6) Increased runoff from extreme storms poses flood risks and can increase erosion and landslides. Even under drier climate futures, extreme rainfall events will occur and may even be more intense than historical events. Flooding is a natural part of fluvial systems, but poses risks to human infrastructure in floodplains. Flood control projects, especially hard infrastructure like levees, channelization, and bank protection structures can have negative effects on habitat. More natural methods of flood management may provide opportunities for maintenance and enhancement of habitat. Over time, stream geomorphology will readjust to changing flow frequencies and intensities but the readjustments can be shocking to the systems (large sediment pulses and redistribution in particular). Where flood protection is important, conservation lands can play an important role in flood attenuation in concert with conservation goals.
7) Decreased recharge will lead to lower base-flow in streams during the dry season, and may turn permanent streams intermittent over much of their length. Protection and management of permanent reaches of streams, such as deep pools, becomes more important.
8) Inter-annual variability in all these factors will remain high or even increase with climate change. Many strategies and tactics to deal with inter-annual (and intra-annual) variability will take on more importance in a “flashier” climate.

All of these changes amplify current management challenges. The triggers for changes have been and will be multi-year droughts or extreme flood events, and fires (the vast majority of ignitions are human caused, but occasional dry lightning storms can cause multiple ignitions in a short period). Identifying landscape-level changes that are a direct result of climate change is a challenge, given the number of natural processes (i.e. succession) and human perturbations (i.e. nitrogen deposition, fire suppression, and invasive weeds) that affect ecosystems. Systematic monitoring of vegetation and aquatic conditions is essential to tease out the contributions and interactions among these drivers of ecosystem change.

All of the principles of effective land, watershed, and riparian management are still applicable, and increase in importance under a changing climate.

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