Modelling water balance in fertilised and unfertilised Cupressus lusitanica and Pinus radiata grown across an environmental gradient

As erosion control and water conservation are almost always important issues, information on water use between different plantation species may be of significant management value for guiding decisions on species selection. The main and interactive effects of species and fertilisation on annual water loss and drainage in three-year old Cupressus lusitanica Mill. and Pinus radiata D. Don were examined across an environmental gradient at eight sites in New Zealand using small highly stocked plots designed to compress the rotation length. Annual average fractional available volumetric water content (θa), total annual water loss and total annual drainage for each treatment at each of the eight sites was determined using a daily water balance model, validated against two-monthly measurements of θa.Modelled annual average θa, total annual water loss and total annual drainage were all significantly influenced by site and species (P < 0.001 for all variables), but not fertilisation or the interaction between species and fertilisation (P > 0.05 for all variables). Averaged across the eight sites, annual average θa for C. lusitanica was higher than that of P. radiata by 0.15 (0.58 vs. 0.43). Total annual water loss for P. radiata was on average 104 mm higher, than that of C. lusitanica (893 mm vs. 789 mm), while total annual drainage for C. lusitanica was on average 129 mm higher than that of P. radiata (285 mm vs. 156 mm). Combining average values for drainage and water loss indicated that there was on average 233 mm more available water per annum in soils under C. lusitanica than P. radiata, which equated to an average of 22% of total annual rainfall.