Question

Bodie Mark Twain Mono Lake Ellery Lake Mono Basin California - Nevada Cain Ranch Benton Gem Lake O z mi N Figure 1.3 - Precipitation stations in and around Mono Basin

Answer 1

1. Average method:

In this method, each point inside the basin is awarded equal weightage regardless of the area of the basin they represent. Thus, only points within the basin are considered. Bodie, Benton and Mark Twain do not lie within the basin and should not be considered for this. The points considered and the average precipitation is given below.

Station	Average annual precipitation (cm)	Average annual precipitation(in)	Percent basin
Ellery Lake	0.0	0	15
Gem Lake	53.3	21	10
Mono lake	34.3	13.5	35
Cain Ranch	28.1	11.1	40
Average rainfall for basin	28.9	11.4

2. Thiessen polygon method:

The most accurate way to draw Thiessen polygons is to draw lines joining the points and connect the midpoints of these lines. Extrapolate them as straight lines towards the edges of the basin if no midpoint is available. After this, use a graph paper to trace these polygons and calculate the number of squares each polygon occupies. Divide this number by the total squares and you'll get the percent area (after multiplying it by 100).

Since I cannot use a graph paper for this (I am doing this on screen), I have used visual estimations. I recommend you to use the graph paper following the above procedure to get accurate numbers. I have drawn the approximate Thiessen polygons below:

Bodie Mark Twain O Mono Lake Ellery Lake Mono Basin California - Nevada Cain Ranch Benton Gem Lake z mi N Figure 1.3 - Precipitation stations in and around Mono Basin

Percentages are presented in the above table.

The average weighted precipitation = Sum of (average precipitation in basinxweight of basin)

= (0x0.15)+(21x0.1)+(13.5x0.35)+(11.1x0.4)= 11.265 inches.

Similarly in cm, the average is 28.579cm.

These averages are slightly lower than those obtained by the average method. The values in inches is smaller by about 0.15 inches, while that for cenitmeters is smaller by about 0.32.