Measuring flow in a small creek using a simple weir

Recently, I constructed a simple weir for measuring flow in Owen's Creek at the point where our ranch-road crosses the creek. There is an old metal pipe, 8' in diameter which the water flows through, under the road. In front of this pipe I laid a rough-cut 2x10 at the base by digging out gravel in the stream bed. On top of this I placed another rough-cut 2x10 with a four foot rectangular notch. By measuring the head (height of water flowing over the notch) one can determine the flow rate of the water in the stream (See table below).

One improvement I want to make is placing a piece of metal in front of the board and up an inch or so from the level of the notch. This will make a thinner area for the water to flow over which gives a more accurate measurement.

There are still some leaks from the bottom of the weir, running about 25gpm (a conservative guess on my part).
Following are some estimates of streamflow using this method, with an estimated accuracy of +/- 30gpm:

July 30th, 2005: 400gpm

September 4th, 2005: 315gpm

September 23rd, 2005: 250gpm

October 1, 2005: 577gpm (1 inch rain in last 24 hours after no rain all summer)

Assuming irrigation rates of 96-146gpm, I will be leaving a downstream flow of 165-215gpm flow (using September 4th measurements).

DISCHARGE FROM A RECTANGULAR WEIR WITH A FOUR-FOOT WIDE CREST GIVEN IN US GALLONS PER MINUTE

Gauge Reading in Inches	GPM
0.5	47
0.75	86
1	145
1.25	199
1.5	258
1.75	320
2	387
2.25	476
2.5	552
2.75	628
3	708
3.25	792
3.5	880
3.75	968
4	1080
4.25	1176
4.5	1268
4.75	1364
5	1472