Sentence:  They are equalled in their productive capacity only by the highest terraces on the slopes of Cerro Gordo in the same region, and their production approaches that  of the irrigated lands in the Lower Valley. The significance of the <WSD>presas</WSD> for agriculture stems from the humidity retention capacity they possess and their ideal location for trapping both soil [...] and water. [...] it is very rare that a crop is lost in the <WSD>presas</WSD> [...] Maize is planted as early as February in some <WSD>presas</WSD>. Special planting techniques are also applied to the <WSD>presas</WSD> to make available subsoil moisture. 
Meaning of the word marked with <WSD> tags:  A temporary dam, dyke or reservoir.