Influence of the thickness and grain size of tephra mulch on soil water evaporation

On the island of Lanzarote (Canary Islands, Spain), under extremely arid conditions - including annual rainfall of below 150 mm - a system for dry farming has evolved based on the use of volcanic mulch. This paper presents the results of the laboratory experiments conducted to assess the influence of two parameters of the mulch - thickness and grain size - on soil water evaporation. A soil typical of the zone, a silty clay Haplocambids, was chosen for the experiment. The mulch cover consisted of medium-grain basaltic tephra in layers 2, 5 and 10 cm thick. A 5 cm thick layer was also studied for fine, medium and coarse basaltic tephra. The soil was saturated and drained until the water content accounted for approximately 50% of weight and it was then subjected to evaporation for 31 days. The evaporation rate was maintained at between 9.1 and 11.5 mm per day, in keeping with an arid climate. The accumulated evaporation in the covered soils, irrespective of the mulch thickness and grain size, was significantly lower than in the uncovered soil. The reduction in accumulated evaporation varied with the mulch thickness: 10 cm of mulch produced a 92% reduction, 5 cm a 83% reduction and 2 cm a reduction of 52%. The 5 and 10 cm coverings provided adequate soil insulation, unlike the 2 cm thickness, which was less effective as a barrier preventing loss through evaporation. All grain sizes reduced evaporation by 81-85%.