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  • Alignment of Learning Goals, Activities, and Assessments
  • Pedagogic Effectiveness
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This page first made public: Jul 27, 2010

Estimating Areal Rainfall in the Lake Whatcom Watershed

Robert Mitchell, Geology Department, Western Washington University

Summary

The learning objectives for this GIS exercise are to 1) understand rainfall spatial variability and how to account for it when making areal-rainfall estimates, and 2) learn how ArcGIS can be used as a tool to determine areal averages, and 3) compare the advantages and drawbacks of the various techniques.

Context

Type and level of course
This is the second out of four GIS exercises used in a senior/graduate level surface-water hydrology course.

Geoscience background assumed in this assignment
Introductory geology and hydrology

GIS/remote sensing skills/background assumed in this assignment
Introductory level GIS experience and cartography skills

Software required for this assignment/activity:
ArcInfo 9.3, Spatial Analyst and 3D Analyst extensions and Arc Toolbox

Time required for students to complete the assignment:
1 to 2 hours

Goals

GIS/remote sensing techniques students learn in this assignment
Students learn how to import point data from Excel, how to apply the Thiessen Polygon tool, the interpolate to raster (IDW) tool, and how to use and apply the raster calculator.

Other content/concepts goals for this activity

Higher order thinking skills goals for this activity
Students must connect how the physics of rainfall formation and the orographic effect influence rainfall spatial variability in high relief topography and the challenges in quantifying areal averages in these circumstances.

Description of the activity/assignment

This is the second out of four GIS exercises used in a senior/graduate level surface-water hydrology course. In the first GIS exercise, students mosaic and resample DEMS and create a mask of the Lake Whatcom watershed using Hydrology tools in Spatial Analysts Tools. The mask is used to clip the DEM of the watershed and other features (e.g., soils and landcover) in subsequent exercises.

The learning objectives for this GIS exercise are to 1) understand rainfall spatial variability and how to account for it when making areal-rainfall estimates, and 2) learn how ArcGIS can be used as a tool to determine areal averages, and 3) compare the advantages and drawbacks of the various techniques. Through lectures and the textbook (Physical Hydrology 2nd Ed. by S. Lawrence Dingham) students learn about the physics of rainfall formation, the orographic effect, spatial and temporal variability, and the limitations of using point measurements to estimate areal rainfall in variable terrain in the Pacific Northwest. They also learn how to apply the Thiessen polygon and inverse-distance weighted methods for estimating areal rainfall averages, and their advantages and limitations.

The Thiessen-polygon method is a traditional weighted-average technique that divides the watershed into polygons, each polygon is associated with a specific rain gauge. In this exercise students use the TheissenPolygon tool in ArcGIS which automatically creates the polygons given x,y locations for the gauges. The areal average is estimated assuming each polygon receives a uniform rainfall depth defined by its respective gauge.

The inverse-distance weighted (IDW) technique takes advantage of the distribution of grid cells in the watershed raster. With the IDW technique, the rainfall on each grid cell in the raster is determined by weighting the distance and rainfall from the surrounding gauges. As such, each grid cell has a different rainfall magnitude. In this exercise students apply the Interpolate to Raster tool (and IDW) in ArcGIS to determine the areal average in the watershed.

There is a lot of rainfall variability in the Lake Whatcom watershed, in part driven by relief and the orographic effect. Although elevations in the watershed reach 1000 m or more, all four rain gauges in the watershed are at or near lake level, approximately 94 m. The Thiessen polygon and the IDW techniques do not account for the change in rainfall with elevation; they assume the entire watershed is at about 94 m. In this exercise, students examine how precipitation increases with elevation using the DEM of the watershed, a simple precipitation lapse rate equation, and the 'raster calculator'.

Determining whether students have met the goals

In the end, students create maps and tables that illustrate and summarize their results. The GIS exercise is part of a writing project that requires an Introduction, Methods, Results, and Discussion. I use their writing as a means to assess their understanding of the science.
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