Perchlorate (ClO4-) reservoirs that accumulated in semiarid unsaturated zones, similar to chloride (Cl-), can contaminate underlying aquifers if they are mobilized. The purpose of this study was to evaluate ClO4- mobilization related to land-use change from natural to agricultural ecosystems in the southern High Plains (SHP, USA), where large ClO4- concentrations (≤60 μg/L) are found in the underlying Ogallala aquifer. Boreholes were drilled and sampled beneath natural ecosystems (3 boreholes) and beneath nonirrigated (rainfed, 7 boreholes) agricultural ecosystems. Large ClO4- reservoirs (361-934 g ClO4-/ ha; peaks 47-111 μg ClO 4-/L pore water), that accumulated for up to ∼30,000 yr under natural ecosystems, are being displaced to depths of 2.2 to >9.2 m in sampled boreholes under rainfed agriculture by increased drainage/recharge. High correlations between ClO4- and Cl- under natural areas (r = 0.81) and rainfed agricultural areas (r = 0.88) indicate that accumulation and mobilization of ClO4- can be predicted from Cl- data. Minimal analysis of ClO4- (e.g., two points, minimum and maximum Cl- concentrations in each profile) can be used to predict ClO4- inventories to within 9% of estimates based on detailed sampling. A pooled linear regression model based on all data in this study (99 points) predicts ClO4- inventories to within 19% of measured inventories. Continued mobilization of pre-existing unsaturated zone ClO4- reservoirs (361-934 g/ha) could increase the current groundwater ClO4- values by a further 8-21 μg/L in the SHP.