Hosts infected with the parasite Cryptosporidium parvum may excrete oocysts on soils in watersheds that supply public drinking water. Environmental stresses decrease the numbers of oocysts after deposition on soils. However, the rates and effects of combined stresses have not been well characterized, especially for the purposes of estimating decrease in numbers. We subjected oocysts to combined stresses of water potential (؊4, ؊12, and ؊33 bars), above-freezing temperatures (4 and 30°C), and a subfreezing temperature (؊14°C) for 1, 14, and 29 days and one to six freeze-thaw cycles (؊14 to 10°C) to estimate coefficients to characterize population degradation using multiplicative error and exponential decay models. The experiments were carried out in NaCl solutions with water potentials of ؊4, ؊12, and ؊33 bars, in combination with temperature stresses at levels that could be expected in natural soils. Increased water potential increased the rate of population degradation for all temperature conditions investigated. Enhanced degradation leads to estimated rates of population degradation that are greater than those that have been reported and used in previous studies conducted to assess risk of water supply contamination from sources of C. parvum.Large proportions of the oocysts of Cryptosporidium parvum shed on soils by infected hosts are probably destroyed by environmental stresses. Important stresses include temperature extremes, freeze-thaw cycling, and extreme water potential (especially desiccation) (20). It is unclear how predictably and at what rate degradation occurs when stresses are combined. Degradation rates are important to understand for the purpose of assessing the likelihood of contamination in the context of public drinking water supply protection. Models developed for the purpose of evaluating the risk of water contamination rely on estimates of coefficients that characterize decay rates before microbial contaminants are entrained and transported to surface waters (13). Risk assessments conducted to evaluate the contamination potential of sources of Cryptosporidium have relied on degradation rates from in situ studies (19). Firstorder decay approaches model Cryptosporidium degradation adequately, especially for conditions related to prolonged exposure to fixed temperatures (9). Present estimates of degradation coefficients are based on investigations of single stresses or uncontrolled combinations of several stresses (9, 16).We tested the hypothesis that the interaction between temperature and water potential stresses enhances oocyst degradation, leading to rates of population decay that are higher than those previously reported and used for risk assessment. We evaluated this hypothesis using exponential decay models, with temperature and water potential stresses represented in a multiplicative error format (8) that related estimates of degradation coefficients to levels of stresses applied. MATERIALS AND METHODSExperimental design. We tested the effects of water potential, temperature, and freez...