Horizontal and Vertical Water Balancing using
Arc Hydro Time Series Data
Alicia Fogg, CRWR
Table of Contents
The South Florida Water Management District (SFWMD) is one of five water management districts within Florida is located on the southern tip of the Florida Panhandle. The mandate of the SFWMD is to provide flood protection and water supply protection to the nearly six million residents who live and work in the area as well manage and restore the ecosystems within the region (SFWMD, 2003). The SFWMD manages a large surface water distribution system in southern Florida, consisting of approximately 1800 miles of canals, over 450 managed structures, and over 1000 flow and stage gages. The SFWMD has currently implemented the Arc Hydro framework to model water movement in the south Florida region. The District has already implement the Arc Hydro Framework Data Model; however, the SFWMD would like to extent the Arc Hydro framework to include additional functionality. One of the avenues of research is developing an Operational Decision Support System (ODSS) which is eventually intended to help Water Managers make scientifically based water management decisions. However, there is no direct link between water contained in the atmosphere (rainfall), on the land, in the ground, and in the canal system; as well as, how water moves between these environments components. The south Florida region is a highly-studied, highly-instrumented area, with many sources of hydrologic information. In order to unify the many different types of hydrologic information available to the Center for Research in Water Resources water volumes, ft3, are used at the unifying unit of measure.
In order to develop the tools to perform horizontal and vertical water balances over the entire SFWMD, a smaller section of the SFWMD was selected. The area selected has been termed the Three Lakes Area, or Prototype Test Area, and includes three of Florida's major lakes, Lake Okeechobee, Lake Istokpoga and Lake Kissimmee. The prototype area will be used to test time-space tools developed at the Center for Research in Water Resources (CRWR).
Location of Three Lakes Area within Florida, detailed view of Three Lakes Area
The general flow of water in the Three Lakes Region is from the North to the South. The flow in this area is currently heavily channelized; however, as part of the Everglades Restoration Project approximately 40 square miles of river ecosystem is being restored to a more meandering channel within floodplains. Within the Three Lakes Area, a smaller basin, C-41A North, was selected in order to develop an understanding of the horizontal and vertical water balances in the region. The C-41A North basin is located just south of Lake Istokpoga, and contains one controlled inlet and two controlled outlets, as shown below.
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Detailed Picture of C-41A North Basin with Three Gated Spillway Control Structures |
Example of Gated Spillway in Prototype Region |
The concept of water balancing is a fundamental concept in water resources planning and management. However, the ability to perform the water balance calculations within the current Arc Hydro framework, and the GIS software platform in general, can be a tedious and labor intensive process. Water movement within a control volume is not only impacted by fluxes, but flows as well. In the horizontal water balance the movement of water through the control volume, in the prototype area: a water control unit or basin, is controlled by structures, which have been represented as a point in the extended Arc Hydro geodatabase. Measurements are collected and stored by the SFWMD as daily-average flow values, which are linked to a point in the geodatabase. In contrast to the horizontal water balance, the vertical water balance is driven by fluxes, which act over areas. Flux time series can be related to many different feature class types, either as points, polygons or rasters. In the SFWMD, rain in the region can be associated with either points, polygons or rasters depending on the measurement type. Rainfall measured at a rain gages is related to a point, rainfall calculated for a rain area (discussed later) is associated with a polygon, and nexrad rainfall grids are associated with rasters. The current difficulty in GIS is the ability to combine all of the inflows and outflows coming into the control volume, regardless of the measurement type, and produce a water balance for the control volume.
Mass Balance Equation for Control Volume:
Linking Control Volumes and Time Series
In order to perform a water balance a control volume and a time period of interest must be selected. It is assumed that the control volume of interest does not change during the calculation period, and thus no time series is associated with the control volume before the calculations. Rather, it is the attributes of water moving feature classes that change over time. In order to link the attributes of the points, lines and areas of interest to the common control volume a Coupling Table is used. The Coupling Table is part of the the TS Viewer tool, which links points, line, areas and grids to a single control volume, a basin. The following is brief description of the components of the Coupling Table. Additional information regarding TS Plotter can be found at: https://webspace.utexas.edu/jgoodall/ArcGISCode.htm.
The horizontal water balance for the C-41A North Basin is controlled by three large gated-spillways that define the edges of the water control unit. A horizontal water balance was conducted on the basin to determine if the landscape was a source or sink for water flowing horizontally through the canal system. All of the flow information was collected using the SFWMD's information database, DBHydro, which contains historic hydrologic and water quality data recorded within the SFWMD. There is limited storage within the canal system, operationally zero storage is assumed in the canal system. Therefore, for this example it is also assumed that no water is stored in the canals themselves; rather, water is either moves through the canal system or is removed from the canal system onto the landscape. The daily storage, in cubic feet per second is calculated using the following formula:
where Qx is the flow from the specified structure, and t is the time period of interest. In this particular case we are interested in the daily value.
Flow into (Blue line) and Out of C-41A North Basin (Red and Yellow Lines) using TS Plotter
Daily Storage Volume for the C-41A North Basin
Over a one year period, from November 1st 2002 to November 16th 2003, an estimated 2.2x1010 ft3 of water entered the basin and an estimated 2.0x1010 ft3 of water left the basin. Comparing the amount of water entering and exiting the basin, approximately 8.5% of the water entering the basin horizontally does not leave the basin in the same time period. An analysis of the canal residence time gives an estimated mean residence time of 1.76 days and a median residence time of 0.76 days, ignoring all days where zero inflow was recorded. This analysis indicates that there is a large amount of water that is either entering the vertical part of the water cycle, or is stored in the landscape. The residence time calculations are based on an estimated canal volume of approximately 4.18x107ft3, and using inflow as the flow. The low residence time in the canal is an indication that evaporation and evapotranspiration within the canal is not a major contributor to water loss; however, evapotranspiration losses within the water control unit may be significant.
The driving factor of the vertical water balance in the south Florida region is rainfall. The spatial and temporal variability of rainfall makes estimating rainfall volumes and vertical rainfall fluxes across an entire basin difficult compared to horizontal aspects of a water balancing, which are well documented in the prototype region. The current operational method used to estimate daily rainfall on each basin within the SFWMD is an estimation method using large areas, called Rain Areas There are approximately 14 Rain Areas that cover the SFWMD, the delineation of each rain area is based on rainfall gage data, Thessien polygons and basin boundaries. Rainfall estimates are then made on a daily basis and provided to the operational managers of the District on a daily basis. The Rain Area estimates for a three month period, from June 1st, 2003 to August 31st, 2003.
Rain Area Estimating Method for the South Florida Water Management District
Operationally the District would like to know if the water from the vertical water balance is entering the canal system in a given basin, or if water is being removed from the canal system into the vertical water balance. Therefore, the objectives of this project for CRWR are: extend the ArcHydro framework to include watershed delineation for the highly controlled flow regime in the prototype area; develop tools to calculate rainfall accumulation and water balances for the watersheds; develop tools to allow water conveyance. These objectives will help answer the questions where is the water, how much is there, and where will/can it go? Rough estimates for the vertical water balance are calculated based on rainfall and potential evapotranspiration. Potential evapotranspiration is currently used for estimating vertical water movement from the land surface to the atmosphere. Comparing the horizontal water storage and vertical water storage, there is a large amount of water storage in the basin over the selected time step. The lack of horizontal water movement into the canal system is a strong indicator that the C-41A North basin is a sink for water in the SFWMD and not a source of water at any point during the year.
Water Volume Storage in the C-41A North Basin
The next step is to close the water balance within the C41-A North canal and other basins within the prototype area. Based on the current analysis, the vertical water balance drives the majority of water storage changes in a control volume. Therefore, additional effort should be focused on developing strong estimates for rainfall volumes, evapotranspiration and runoff into the canal system. The next step is to use nexrad data, supplied by the SFWMD to develop a more accurate estimate for the rainfall volume entering each basin. Currently 2 kilometer by 2 kilometers nexrad grids are collected over the SFWMD. However, the SFWMD does not currently have a direct method of connecting the volume of water associated with rainfall to the landscape. The inclusion of nexrad data in the water balance calculations would decrease the calculation time step from a daily time step to a 15-minute time step. A shorter time step, such as 15-minutes, is eventually where the SFWMD would like to go with this decision support system
Nexrad rainfall grids over the Three Lakes Region from 6:15 am to 12:45 pm June 20, 2003
The questions that SFWMD would eventually like to answer using the ArcHydro based decision support system are:
How much water is flowing into or out of a basin at any given time?
What is the amount of water is all forms within a basin?
What is the remaining storage capacity in a basin or lake?
What are the available routing options for water movement within the District?
How do can different time step resolutions be combined to produce storage and routing solutions?
Prototype Area Geodatabase (ArcGIS 9.0) ODSS3Lakes.mdb
Alicia Fogg
Graduate Research Assistant
Center for Research in Water Resources
Department of Civil Engineering, University of Texas at Austin
(512) 471-0073
These materials may be used for study, research, and education, but please credit the authors and the Center for Research in Water Resources, The University of Texas at Austin. All commercial rights reserved. Copyright 2004 Center for Research in Water Resources.
