Plant productivity is strongly dependent upon moisture. Ecosystem models estimate the moisture content in the air from measurements or vapor pressure. Realistic vapor pressure values allow these models to more accurately estimate plant productivity. Observational measurements of vapor pressure ar... Show morePlant productivity is strongly dependent upon moisture. Ecosystem models estimate the moisture content in the air from measurements or vapor pressure. Realistic vapor pressure values allow these models to more accurately estimate plant productivity. Observational measurements of vapor pressure are relatively scarce, and a reliable means of estimating vapor pressure is needed. CLIMSIM and its later version, MTCLIMv3, are climate simulators that estimate daily vapor pressure. CLIMSIM calculates vapor pressure by assuming that the dewpoint is reached every night and that it is equal to the night-time minimum temperature, a more widely available parameter. Unfortunately, this method overestimates the vapor pressure in arid and semi-arid regions. MTCLIMv3 estimates vapor pressure by scaling down minimum-temperature derived calculations by a ratio of daily potential evapotranspiration to annual precipitation. This study compared the vapor pressure estimates from CLIMSIM and an improved version. MTCLIMv3. The input data needed for the climate simulators, daily maximum and minimum temperature and daily precipitation, were obtained from a WGEN weather generator. CLIMSIM and MTCLIMv3 simulated vapor pressure for six locations in the United States: five in arid and semi-arid regions in the West and one in a relatively wetter region in the East. The success of the most recent version , MTCLIMv3, was determined by its ability to match the average monthly vapor pressures in the Danny Marks data set. which were derived from a climatology and assumed to be accurate. MTCLIMv3 out-performed CLIMSIM in all five of the arid and semi-arid regions. However, for certain months, MTCLIMv3 did no better than CLIMSIM at two dry sites. Why MTCLIMv3 fails to improve vapor pressure estimates all year for some dry climates and only some months for other dry climates needs to be investigated further. Show less