Chapter One Averaging Precipitation over a watershed

The NW windward precip., Western interior rain shadow arid, east dominated by mT versus cP invasions. Gulf Coast very rainy due hurricanes. Red Subtropical desert in summer. Annual Rainfall CONUS lower 48

Seasonal Rainfall The west coast windward precip in fall- winter, snowmelt in spring, east of cordillera arid with May rain, eastern US year round precip +/- mid- year rain.

Precipitation is generally recorded automatically in increments Transmitter Reed switch activated by magnet

For our purposes, a wide can will do

A Hyetograph On a rainy day, rainfall intensity varies with time. Plot called a hyetograph.

Intensity-duration-frequency (IDF) curves. How frequently do you get a storm of a particular intensity and duration? Here is a summary for Houston. Every five years, on average, they get an inch of rain per hour lasting 4 hours. Every 100 years they about half an inch of rain per hour lasting 24 hours.

Notice only the gages within the watershed are used for the average. All gages used There are almost never enough reliable rainfall records inside the basin you are working on. Until Nexrad archives improve, three averaging methods are used: taking the mean, Thiessen method, and the Isohyetal method, similar to making a contour map.

For Thiessen, find the rain gauges near your watershed

Draw triangles between adjacent rain gages and draw perpendicular bisectors for each side

Thiessen Figure E1-4b This breaks up the area around each gauge into regions. CONSIDER ONLY the area of each region that belongs to your watershed. Here the area around gage D is the largest, then B, then F, then A, etc. Measure these areas by drawing a grid onto the map, by tracing/printing them on gridded paper, or by some other method.

Add all the small grid areas together to get the total area of the watershed, then calculate the fractional area for each gauge. Multiply the proportionate area for each polygon by the rainfall for that gage, and add them up for the weighted average total rainfall over your watershed.

NEXRAD (Next-Generation Radar) gives you fine resolution data with area squares. These are available real time (current weather), but the online archives are still unwieldy for past storm events.

Thiessen Method We will stop here to work on averaging and Theissen method.