Monte Carlo simulations and a combination of site-specific data (e.g., soil properties, climatic conditions, and native vegetation) were used to design alternative (evapotranspiration) landfill covers at Edwards Air Force Base, located near Lancaster, CA. Laboratory analyses of site soils indicated the presence of three distinct surface soils, from which statistical distributions were generated. A 10-yr climate sequence (precipitation and potential evapotranspiration) was used for the upper boundary. Potential evapotranspiration was partitioned into potential evaporation and potential transpiration using the phenology of a Mojave Desert plant community. Nearly 1000 realizations were run for each of 72 different combinations of soil type, cover thickness, and plant cover percentage. The results indicate that threshold design parameters, needed to limit deep flux to <0.5 cm yr−1, differ based on the relationship between the Ks (saturated hydraulic conductivity) of the surface soil, cover thickness, and plant cover percentage. In the lower conductivity soils (mean Ks = 20 cm d−1), deep flux was ≤0.2 cm yr−1 for a cover thickness >80 cm with a plant cover >10%. Higher conductivity soils (Ks = 250 cm d−1) required thicker soils covers (>100 cm) and greater plant cover (>20%) to achieve similar fluxes. In all cases, variations in both cover thickness and plant cover percentage indicated threshold values, above which incremental additions added little to cover performance. The methods developed here could be implemented at other sites where conditions are known. Designs can account for uncertainties in site parameters and contribute to improved decision making.