What do we understand by hydrological applications of radars?




Could we play Hidrology with Radars?

Presentation by Daniel Sempere Torres at the "International Conference on Advances in Flood Forecasting in Europe" in Rotterdam, 3-5 of March, 2003

The weather radar is nowadays the only instrument able to give a detailed description of the spatial and temporal rainfall field. This information is needed to hydrologically model the Mediterranean region with sufficient resolution.

However, the radar is a complex instrument. It measures a property of the rainfall drops. This property is related to the portion of the power of the beam put out by the radar and that returns to it once the beam has hit its target. This property, the rainfall reflectivity (Z), is indirectly related to the rainfall intensity (through the raindrop sizes distribution). It is also indirectly related to the intensity of the rainfall that reaches the ground (which is, what is of interest from a hydrology point of view).

Due to the factors mentioned above, measuring rainfall through a radar requires the physical comprehension of the meaning of the measure itself and of the factors that affect the precision of the transformation of the the rainfall intensities at ground level. The effort on this area has been postponed since the 40s, when the weather radar was developed. The goal was to do research on radar technology in order to be able to use the radar promptly for meteorological forecasting.

Nowadays, the greatest, and added value that the radar has, is the possibility of using it for modeling purposes and for water management. However, a significant research and development effort needs to be done in order to design and develop algorithms valid for the climatologic conditions of the Mediterranean thunderstorms, algorithms that will allow the optimization of the quantitative estimation of the rainfall intensity.

The effort that needs to be invested in research and development is great, but its reward is however very promising: To have a rainfall field every 5 to 10 min with a precision of one measure every km2 over the area covered by the radars. This precision is much greater than the one provided by the traditional rainfall measuring methods. Each of the automatic teletransmitting raingages in the "SAIH" network (Automatic System of Hydrologic Infrastructure) Hidrològica) covers an average area of 500 Km2 in Spain. The situation is better for the Catalonia Internal Watersheds (Oriental Pyrenees) where each raingage covers an average area of 200 Km2. This required an unheard of effort in the hydrologic history of Catalonia.

This attractive prospect of the radars provides additional rentability to the network of 13 radars that INM has (plus two more that are being installed) that cover a great part of Spain and to the network of 4 radars SMC of Catalonia. Three are already installed and one will be shortly installed (2004).

Our goal since 1994 has been to develop hydrologic applications for the radar. Our research has been financed through a series of "I+D" (Reserach and Development) projects. These projects allowed us to initiate a pioneer research line in Spain and to set the foundation to what we expect will be a very profitable path for the development of hydrometeorology in our country.