Snowpack depth modelling and water availability from LIDAR measurements in eastern Pyrenees Ignacio MORENO BAÑOS (1)* , Antoni RUIZ GARCÍA (2) , Jordi MARTURIÀ I ALAVEDRA (1) , Pere OLLER I FIGUERAS (1) , Jordi PIÑA IGLESIAS (1) , Carles GARCIA SELLÉS (1) , Pere MARTÍNEZ I FIGUERAS (1) , Julià TALAYA LÓPEZ (2) (1) Institut Geològic de Catalunya, C/ Balmes, 209-211. E08006-Barcelona . *imoreno@igc.cat (2) Institut Cartogràfic de Catalunya, Parc de Montjuïc - 08038 Barcelona. ABSTRACT: In order to evaluate water reserves in mountain watersheds the Institut Geològic de Catalunya (IGC) jointly with Institut Cartogràfic de Catalunya (ICC) have begun a project to model snowpack depth distribution at the study site of Vall de Núria (38 Km 2 basin located in eastern Pyrenees). Re- mote sensing airborne LIDAR (Light Detection and Ranging) survey and field work validations were performed to make this calculation. Modelling snowpack distribution is a complex task because of its spatial variability. Despite being a recently developed technique, LIDAR has become a useful method in snow sciences because it has the advantage to offer dense point data and to cover wide areas with little economic and field work effort. The new methodology presented combines LIDAR data with field work, the use of Geographical Information Systems (GIS) and the stepwise regression tree (SRT), as extrapolation technique has allowed us to map snowpack depth distribution with spatial high resolu- tion. Extrapolation is necessary because raw LIDAR data is only obtained from part of the study area in order to make the technique as affordable as possible. Promising results show low differences of total snow volume calculated from modeled snowpack distribution and total snow volume from LIDAR data only differ 1.4%. KEYWORDS: snowpack depth, stepwise regression tree, LIDAR, GIS, Pyrenees 1 INTRODUCTION Mediterranean climate is characterized by a high pluviometric variability. As a result of this variability the Iberian Peninsula, and Catalonia as part of it, are affected by frequent and serious droughts that alter the availability of water sup- ply. Hydric resources are not only concerned by natural fluctuations, but social pressure must also be taken into consideration. Catalonia’s internal fluvial basins (those, whose responsibil- ity is the local Government) occupy 52% of total Catalonia’s surface but houses the 92% of the total population (Sangrà, 2008) which increases hydric stress. As a consequence of particulari- ties mentioned above it is necessary to quantify hydric resources stored as snow. High snowpack variability (Elder, 1995) and sparse snow depth data make it difficult to mod- el snow cover. For this reason remote sensing is essential when modelling wide areas. The use of airborne LIDAR (Light Detection and Ranging) to model snow depth has the ad- vantage of covering large areas with high reso- lution at a relatively low economic cost. The ap- plication of LIDAR to model snow depth is pos- sible due to the high accuracy it provides, verti- cal error of 15 cm in ideal conditions, as several studies have shown (Hopkinson, 2001; Fassnacht, 2005; Deems, 2006) The present study, Use of LIDAR to evalu- ate water reserves stored as snow in mountain watershed, is carried out by Institut Geològic de Catalunya (IGC) jointly with Institut Cartogràfic de Catalunya (ICC). The project is composed of several stages with the final aim being to model water availability in mountain watersheds. First of all, snow depth volume was modeled and the results of this first stage are presented here. A pilot study site at Vall de Núria has been set u`p in order to validate LIDAR technique and to establish a valid methodology to model snow depth over large areas. The valley itself covers an area of 38 km 2 with altitude ranging between 1950m at Núria Sanctuary and 2910m at the summit of Puigmal peak. Most part of the surface is above timber- line with meadows and rocky soil covering most of the area that makes drifting snow really fre- quent. ______________________ Corresponding author address: Moreno, I C/ Balmes, 209-211. E08006-Barcelona, Spain ; Tel. 34 93 553 84 30 (2518); email: imoreno@igc.cat