These credit systems aim to quantify the services provided by trees to treat stormwater and preserve water quality, air quality, and carbon sequestration.[2][3] The credits allow site designers to subtract area from total site area or impervious area when calculating water quality volume (WQv) and/or recharge volume (REv). Credits are not calculated consistently across these systems due to the fact that tree benefits can vary with size, species, climate, site etc. Two common types of credit are impervious surface reduction and volume reduction.