GIS Hydro '99: Atmosphere and Soil Water

Atmosphere and Soil Water Module

Center for Research in Water Resources, University of Texas at Austin

View an animated map of moisture flow over North America (June, 1991 - July, 1993)

Introduction

This page describes research and educational material related to water balance calculations in the atmosphere and in the soil. Atmospheric water balance refers to the computation of moisture changes in a large region by monitoring the moisture fluxes across the boundaries of the region. Soil water balances on the other hand involve computing moisture changes in a region by monitoring or computing the moisture fluxes in and out of the soil. If horizontal moisture fluxes in the soil can be neglected or accounted for, the two moisture balances can be shown to provide independent measures of the same quantity.

In addition to previously presented masters theses by Seann Reed and Kris Martinez, and the Spatial Water Balance of Texas report, a PhD dissertation detailing the research of Seann Reed in developing a GIS based Soil Water Balance Modeling System is presented. The atmospheric water balance remains unchanged. Two exercises are used to guide the user through the steps required to perform an atmospheric water balance.

Atmospheric Water Balance

Definition:
Define a control volume in the atmosphere bounded vertically by the earth's surface and the top of the atmosphere (defined at a specified atmospheric pressure) and horizontally by an arbitrary polygon (i.e. a state or country boundary) extended from the earth's surface to the top of the atmosphere. Use radiosonde data or the output data from a General Circulation Model to compute the inflow and outlfow of atmospheric moisture. If data are also available to compute the change in atmospheric water storage or if the change in atmospheric water storage is assumed negligible, regional runoff can be estimated using this approach. If precipitation data are also used, estimates of regional evaporation can be made.

Atmospheric moisture flow visualization

Exercise on the 1993 Midwest Flood
This exercise takes a generalized border of the Upper Mississippi Basin and part of the Missouri Basin and does an atmospheric water balance using 12 hour time intervals from March to July 1993, during the great flood which occurred in the Midwest at that time.

Exercise for processing a boundary
This exercise takes the polygon boundary of Texas, approximately 7000 km in length and originally consisting of about 25,000 line segments, and simplifies it into approximately 70 line segments of about 100 km each so that atmospheric moisture flow calculations can be done segment by segment in a reasonable way. This exercise can be used to prepare boundary line data for water balance calculations like those done for the 1993 Midwest Flood Exercise (above).

• Report describing methodology in detail: Spatial Water Balance of Texas (See Chapter 3)

Soil Water Balance

Definition:
Using a soil layer as the control volume, estimate the amount of water entering and leaving the soil, and keep track of the amount of water stored in the soil. Use parameterization schemes to estimate runoff and evaporation given precipitation and other meteorological data.

• Reports
• Reed, S.M., and D.R. Maidment, Use of Digital Soil Maps in a Rainfall-Runoff Model, CRWR Online Report 98-8, December 1998. This is a documentation of Seann Reed's PhD dissertation research into the development and testing of a methodology for continuous soil moisture accounting over large areas. The model uses soil parameters derived from standard USDA soils databases, and climatic data from NWS NEXRAD precipitation data and NASA surface radiation budget data.
• Reed, S.M., D.R. Maidment and J. Patoux, Spatial Water Balance of Texas, CRWR Online Report 97-1, February 1997. This report details the use GIS for a statewide water balance study. Chapter 4 of this report details the methodology used for the soil water balance component of the study.
• Reed, S.M., and D.R. Maidment , A GIS Procedure for Merging NEXRAD Precipitation Data and Digital Elevation Models to Determine Rainfall-Runoff Modeling Parameters, CRWR Online Report 95-3, September 1995. This report details a GIS based procedure for transforming NEXRAD precipitation data from to runoff at a watershed outlet. It also describes the issues involved in transforming inputs from the spherical datum used by NEXRAD cell to the ellipsoidal datum used in DEMs.
• Martinez, K.L., and D.R. Maidment, (1998), Assessing the Impact of Climate Change on Water Resources for the Edwards Aquifer. It describes Kris' research into the effects of climate change on the Edward's Aquifer in Central Texas. In this study, a simple soil water balance model is linked to a recharge and groundwater model of the aquifer.
• Presentations
• A Soil Water Balance and Continuous Streamflow Simulation Model by Seann Reed describing the development of a Soil Water Balance Modeling System and its use in studying the effect of spatial complexity on modeling accuracy.
• Impact of Climate Change on Water Resources for the Edwards Aquifer by Kris Martinez describing a GIS based analysis of long term water levels in the a major aquifer in Texas.

Coupled Atmospheric and Soil Water Process Models

• Preparing data sets for a land surface-atmosphere model using GIS: "Evaluating the Technique of Variational Assimilation in a Semi-Arid Environment" by Karen Mohr.

Primary Contact

David R. Maidment
University of Texas at Austin
Center for Research in Water Resources
Austin, Texas 78712

e-mail: maidment@crwr.utexas.edu
Telephone: (512) 417-0073
Fax: (512) 471-0072

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 1999 Center for Research in Water Resources.