The Community Collaborative Rain, Hail and Snow network (CoCoRaHS), www.cocorahs.org, is filling a practical niche in national-scale water balance observations that may contribute to improvements in land surface evapotranspiration estimates. CoCoRaHS volunteers are distributed across the entire country measuring and reporting precipitation on a daily basis from nearly 10,000 observation points. Data are available promptly each day as reports are submitted and are quality controlled.

This data set complements and greatly extends the data available from official National Weather Service ground stations providing higher spatial resolution than most other in situ precipitation measurements and providing an excellent data resource for precipitation mapping or for calibration and ground truthing of remotely sensed data.

In 2011, CoCoRaHS began pilot testing a low cost evapotranspiration measurement using the ETgage. The ETgage is a modified atmometer with a green canvas/wafer covering the ceramic evaporating cup to simulate crop albedo and vapor diffusion resistance.

With reference ET estimates from the ETgage, spatial patterns of evaporative demand can be added to the already detailed maps of precipitation. Examples of water balance calculations from the CoCoRaHS ET pilot project will be shown. Plans for deploying a nationwide network of ETgages will be described.