Friday, January 23, 2015

Effective rainfall explained ( with actual rainfall charts!)

There is a lot of confusion surrounding using weather reports and historical rainfall ( backyard or online rain gauges )and how much rain actually soaks into the soil profile and goes deep into the root zone of plants and becomes plant available water (PAW.) Rain that gets absorbed into the soil in the exact place it landed is call this EFFECTIVE RAINFALL.

The rule of thumb for scheduling irrigation is to take 75% of all rainfall as effective. On a Day in which we get a nice light rain, few an far between in Central Texas,  75% can come close to reality, but for most of central texas in flash flood alley, this is overly optimistic.

Below is a chart of Wednesday Jan 21 from about 4pm to 9am  Thurday Jan 22nd. We received a total ACTUAL rainfall over an 18 hours period of 2.23 inches ( $5,000 weather gauge measured at LCRA Redbud Center so pretty accurate). The gauge measures and transmits in 15 min increments, so we were able to calculate EFFECTIVE RAINFALL using typical absorption rate of 1/4" per hour ( or 1/16" per 15 min interval ) for clay loam soils. The calculated EFFECTIVE RAINFALL is 1.55 inches. 

The RED line is water that went into the ground where it landed and was effective. The BLUE peaks are essentially runoff, the water may pool somewhere and not run into the street, but is is not in the same place it landed.

The Effective rainfall comes in at 70% of actual rainfall which is pretty much the highest percentage we can ever expect in Central Texas.  When we see rainfall rates as high as 4 or 5 inches per hours, it is not uncommon to see effective rainfall percentages in the 10-25% range.

If we get back-to-back rain storms and the soil is already saturated ( a term sometimes called FIELD CAPACITY ) , we may not absorb as much of the rainfall rainfall that is considered "effective."

Thanks and happy pondering. Cougar Matt out.

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