Calculation of High Resolution Hydrology in the ATLSS Modeling System November 1997 Michael Huston and Scott Sylvester The Institute for Environmental Modeling University of Tennessee - Knoxville Copyright 1998 - The University of Tennessee One of the key features of the ATLSS models is the use of a high resolution hydrology model based on vegetation maps to convert the low resolution hydrologic output of the South Florida Water Management Model (1 water value per 2 x 2 mile cell) to the higher resolution needed to model ecological processes and the distribution of wildlife species. The ATLSS High Resolution Hydrology Model post-processes the output of the South Florida Water Management Model (WMM) using an algorithm based on conservation of water volume, and redistributes the water volume over a surface of high resolution topography (ATLSS "pseudotopography") to produce a high resolution map of water depth. This process is repeated on daily timesteps (corresponding to the daily output of the WMM) to create a map of water depth across the wetlands of South Florida with over 3000 separate values within each 2 x 2 mile cell (using topography based on the 28.5 meter resolution of a LandSat image). The first step in calculating high resolution hydrology is processing the output of the Water Management Model. This output is provided as daily values of water level for each of the approximately 1700 2 x 2 mile cells of the WMM. For the simulation period of 1965-1995 this output is an 88 megabyte file. These data are provided as distance of the water surface from the ground surface (+ or -) and so require a separate file of surface elevations to be converted to absolute elevation above mean sea level. These water surface data are converted to water volume per 2 x 2 mile cell by setting the basal surface of the volume occupied by water in each cell at an elevation 20 meters below the ground surface elevation. This elevation was selected because analysis of WMM output for the 31-year simulation period showed that in no cell did the water level ever exceed 20 meters below ground surface, nor 20 meters above ground surface. Thus, each cell has a base elevation for calculation of water volume that is set in relation to its surface elevation. This base elevation is constant throughout the 31 year period. The 40 meter range allows water surface elevations to be stored at 1 millimeter resolution using a "short integer" format in C++. Because the water surface elevation can be either above or below the ground surface in any cell, water volume calculations must be subdivided into below and aboveground components. Above the ground surface, standing water occupies 100% of the volume defined by the following equation: volume = (water level - ground surface) x the surface area of the cell. Below the ground surface, ground water occupies only a fraction of the total volume, since sediment or bedrock occupy most of the volume (that is, if the ground is "solid"). In the WMM, the "water storage capacity" of the bedrock is modeled as varying from region to region across South Florida, but most values are close to 0.2. Thus, ground water is 20% of the volume defined by the following equation: volume = (groundwater level - basal elevation) x the surface area of the cell. When the water level is above the ground surface, calculation of total water volume has both an aboveground and a belowground component. The ATLSS High Resolution Hydrology Model calculates the volume of water predicted by the WMM for each 2 x 2 mile cell for each day of the 31-year simulation period. The primary WMM output file used is: daily_stg_minus_lsel.bin. The WMM input file from which the bedrock water storage capacities are obtained is: statdta.int95_2. WMM data values in feet are converted to metric for use in ATLSS. Thus, the 31 year file of daily water level output is converted to a 31 year file of daily water volume in each of the 1700 cells of the WMM "Map" of South Florida. The second step in calculating high resolution hydrology is redistributing the water volume for each 2 x 2 mile cell over the irregular topographic surface of the ATLSS pseudotopography. Pseudotopography is used to replace the completely flat surface of the WMM 2 x 2 mile cell with an undulating surface that corresponds to the topography underlying the vegetation (i.e., the highest areas are hardwood hammocks or pine stands, and the lowest are open water or deep marsh vegetation). The same basic subdivision of volume calculations by surface and subsurface water that was used to calculate water volume from the water surface elevations of the WMM is used to convert the water volumes back into surface elevations of water on the undulating landsurface of the pseudotopography. The water surface is assumed to be level across each 2 x 2 mile square of pseudotopography landscape. Consequently, for any particular water surface elevation, some fraction of the total water volume will be surface water (assuming the water level is above the lowest point on the land surface) and the rest will be subsurface water. The ATLSS High Resolution Hydrology Model calculates the water surface elevation for any specific water volume by "balancing" the surface and subsurface volumes to equal the total water volume produced by the WMM. This "high resolution hydrology" can potentially estimate water depths for each 28.5 x 28.5 meter area within the region covered by the WMM, rather than a single water depth for each 2 x 2 mile area. Thus, the ATLSS High Resolution Hydrology Model converts a low resolution map of 1700 surface water elevation values into a high resolution map with as many as 5.5 million surface water elevation values within the same total area. The ATLSS High Resolution Hydrology Model provides water depth estimates at spatial scales that are relevant to the vegetation and wildlife species of the Everglades and Big Cypress. Nonetheless, this level of resolution (28.5 m) is not fine enough to detect certain biologically important features, such as alligator holes. For other purposes, such a high resolution is not necessary. Consequently, some of the ATLSS animal models use water data that has been aggregated to 100m or 500m cells, which are based on averages of the 28.5 meter data. Potential Errors in High Resolution Hydrology Calculations There are three primary sources of error in the daily water depths calculated by the ATLSS High Resolution Hydrology model: 1) Errors in the pseudotopography base map used to calculate water depth; 2) Errors in the daily stage height output of the Water Management Model; and 3) Discrepancies between the structure of the WMM and the actual physical structure of the South Florida Landscape, as reflected in the vegetation map and the derived pseudotopography. Errors in the base pseudotopography can result from several sources. Obviously, errors in the satellite-based vegetation map will result in errors in any products derived from that map. However, while this map certainly contains a number of misclassified 28.5 x 28.5 m cells, the overall pattern of vegetation is consistent with other maps of South Florida vegetation. So these errors should have relatively little impact on the overall validity of pseudotopography. A second source of error is the hydroperiod parameters used to generate pseudotopography from the vegetation map. These parameters are based on values reported in the literature, and are consistent with the general topographic positions of the major vegetation types found throughout the South Florida wetlands. More serious sources of error for both pseudotopography, and all water calculations are 2) and 3), listed above. Wherever the output of the WMM does not closely match the actual hydrograph that occurs (or would occur) in a particular location, both pseudotopography (based on the "calibration-validation" runs of the WMM) and high resolution hydrology may be incorrect at that location. It is important to note that there is a possibility that high resolution hydrology (HRH) may in some situations actually decrease the errors present in WMM output. This occurs because HRH, specifically the underlying pseudotopography, force the shape of the landscape to create hydroperiods appropriate for the vegetation types that are present, while it preserves the water volume predicted by the WMM. For example, where the WMM predicts a long hydroperiod in an area where the vegetation maps shows only short hydroperiod vegetation types (such as pine and Muhlenbergia), the HRH pseudotopography algorithm will raise the surface of the landscape sufficiently that the appropriate hydroperiods will be experienced by the vegetation. In such a case, the bulk of the water volume will be stored as subsurface water. Errors in WMM output are particularly critical where they result in predicting more favorable conditions than those that would actually occur. This would cause all derivative model runs to overpredict the population density of affected species. In critical habitats, it is important that WMM output (specifically the calibration-validation runs on which all empirical model validation will be conducted) be checked against measured stage height data. The third major source of error results from the inevitable mismatch between a relatively coarse scale model (the 2 x 2 mile grid structure of the WMM) and the actual fine-scale patterns of vegetation and control structures on the landscape. The grid structure of the WMM requires that physical structures, such as levees, be represented as occurring along the edges of the 2 x 2 mile cells, which produces a "stair-step" approximation of the actual linear structure in the wetlands. This creates a problem for HRH when vegetation on the "down-gradient" side of a levee within a 2 x 2 mile cell is modeled as having water based on the "up-gradient" side of the levee. This problem occurs primarily along the boundaries of the Water Conservation Areas (and is discussed in the file on the Boundaries of the Reporting Units used by ATLSS). This problem results in HRH being incorrect in those areas where WMM boundaries do not coincide with actual levees and/or canals. To some degree, HRH and pseudotopography adjust for this type of misalignment by alterning the topographic surface of the landscape so the vegetation experiences the appropriate hydroperiod under calibration conditions. However, this results in a discrepancy between the water depth and hydroperiod predictions of the WMM raw output and the water depth and hydroperiods predicted by the High Resolution Hydrology. In such areas of discrepancy, there is a good probability that the HRH results are more accurate that the raw WMM output, but this should be checked against actual measurements of topography and hydrographs wherever possible. One potential solution to the problem of the mismatch between WMM model boundaries and the actual boundaries on the landscape is to post-process the WMM output into the irregularly-shaped cells that correspond to the areas of discrepancy. This would be a substantial programming effort, but would solve this major source of error for all future model runs. ===================================================================== Performance measures associated with the ATLSS Hydrology model. In accordance with the ATLSS file naming conventions, each file name will consist of the characters: "X" or "_" => the Base, typically F for the F2050 base or E for the C1995 base "X" or "_" => the alternative scenario or base "HY" => the ATLSS Hydrology Model "XXXX" => 4 character mnemonic "." "PDF" or "TXT" or "DOC" => PDF, tabular text or documentation 1. Maps The comparison maps for the ATLSS Hydrology model reflect a graphical representation of the hydroperiod data found in the accompaning tables. For a detailed discussion of the hydroperiod data, see the description of the comparison tables below. The comparison maps associated with the ATLSS Hydrology model consist of the following files: XXHYHCM1.TXT Comparison map set showing the total area by hydroperiod classes averaged over the years 1985 to 1995 grouped by subregion. XXHYHCM2.TXT Comparison map set showing the total area by hydroperiod classes averaged over the five driest years grouped by subregion. XXHYHCM3.TXT Comparison map set showing the total area by hydroperiod classes for the driest year grouped by subregion. XXHYHCM4.TXT Comparison map set showing the total area by hydroperiod classes averaged over the five wettest years grouped by subregion. XXHYHCM5.TXT Comparison map set showing the total area by hydroperiod classes for the wettest year grouped by subregion. XXHYHCM6.TXT Comparison map set showing the total area by hydroperiod classes for the average year grouped by subregion. ======================================================== 2. Time Series Charts None. ======================================================== 3. Histograms None. ======================================================== 4. Tables The high resolution hydrology analysis for the Across Trophic Level System Simulation ( ATLSS ) consists of a set of tables which compare two hydrology scenarios as provided to our research group by the South Florida Water Management District ( SFWMD ). The comparisons are broken down into two basic types of files. The first type contains hydroperiod analysis for the vegetation types. The other contains an report on the area in each of 10 hydroperiod classes. These analyses are repeated for each of several regions in South Florida (SF) and for a variety of years or combinations of years. The comparisons are carried out on a variety of subregions of SF. These regions represent major control and management areas of SF, such as Loxahatchee, the water management areas, and management regions with in The Everglades National Park ( ENP ) and Big Cypress National Preserve ( BCNP ). The comparisons are also broken in to a number of temporal units. The current time groupings are a ten year average from 1985 to 1994, an average of five years which had the highest rain fall ( currently 1966, 1968, 1969, 1982, 1983 ) an average of five years which had the lowest rain fall ( currently 1971, 1981, 1988, 1989, 1990), the year with the highest rain fall ( 1969 ) the year with the lowest rain fall ( 1990 ) and the year with an average rain fall ( 1977 ). The first type of file provides hydroperiod comparison for the vegetation type. Within each region the average hydroperiod is computed for each vegetation type which covers at least 10% of the area of that region. At the top of each table is the name of the region and the total area of that region. Each row of a tables represents the values for a single vegetation type. The columns of each row are defined as follows: The vegetation type index and description, the area of the region covered in that vegetation type, the hydroperiods for the vegetation type under the two scenarios, the difference between the hydroperiods and the percentage difference. The indices represent those used in the Pearlstine vegetation cover map to represent the vegetation types. The vegetation type descriptions are those used by University of Florida Land Cover Classification map developed in support of the Florida Gap Analysis Program (FGAP). Hydroperiods have units of days, and the areas are listed in Km. The second type of table gives the area covered in each region by each of twelve hydroperiod classes. The top of the table gives the region name and the total area of the region. Each row of the tables contains the information about a single hydroperiod class. The columns of each row are defined as follows: The first column contains the range of hydroperiod which defines the hydroperiod class, the next two are give the area of the listed region which has a hydroperiod with in the range of values for that class for each of the scenarios being compared and the last column gives the difference between these two values. The mnemonic section of the file names are composed as follows: "XX" = P1, P2, P3, P4, P5 => the Time Period "XX" = HC, VT => Hydrology Class, Vegetation Type The performance measure tables associated with the ATLSS Hydrology model consist of the following files: File Name Description ------------ ------------------------------------------------ XXHYVTP1.TXT Comparison tables set showing the total area of significant vegetation types averaged over the years 1985 to 1994 grouped by subregion. XXHYVTP2.TXT Comparison tables set showing the total area of significant vegetation types averaged over the five driest years grouped by subregion. XXHYVTP3.TXT Comparison tables set showing the total area of significant vegetation types for the driest year grouped by subregion. XXHYVTP4.TXT Comparison tables set showing the total area of significant vegetation types averaged over the five wettest years grouped by subregion. XXHYVTP5.TXT Comparison tables set showing the total area of significant vegetation types for the wettest year grouped by subregion. XXHYVTP6.TXT Comparison tables set showing the total area of significant vegetation types for the average year grouped by subregion. XXHYHCP1.TXT Comparison tables set showing the total area by hydroperiod classes averaged over the years 1985 to 1995 grouped by subregion. XXHYHCP2.TXT Comparison tables set showing the total area by hydroperiod classes averaged over the five driest years grouped by subregion. XXHYHCP3.TXT Comparison tables set showing the total area by hydroperiod classes for the driest year grouped by subregion. XXHYHCP4.TXT Comparison tables set showing the total area by hydroperiod classes averaged over the five wettest years grouped by subregion. XXHYHCP5.TXT Comparison tables set showing the total area by hydroperiod classes for the wettest year grouped by subregion. XXHYHCP6.TXT Comparison tables set showing the total area by hydroperiod classes for the average year grouped by subregion.