Abstract
Summerfallow is cropland that is purposely kept out of production during a growing season to conserve soil moisture. On the Canadian Prairies, a trend to continuous cropping with a reduction in summerfallow began after the summerfallow area peaked in 1976. This study examined the impact of this land-use change on convective available potential energy (CAPE), a necessary but not sufficient condition for moist deep convection. All else being equal, an increase in CAPE increases the probability-of-occurrence of convective clouds and their intensity if they occur. Representative Bowen ratios for the Black, Dark Brown, and Brown soil zones were determined for 1976: the maximum summerfallow year, 2001: our baseline year, and 20xx: a hypothetical year with the maximum-possible annual crop area. Average mid-growing-season Bowen ratios and noon solar radiation were used to estimate the reduction in the lifted index (LI) from land-use weighted evapotranspiration in each study year. LI is an index of CAPE, and a reduction in LI indicates an increase in CAPE. The largest reductions in LI were found for the Black soil zone. They were −1.61 ± 0.18, −1.77 ± 0.14 and −1.89 ± 0.16 in 1976, 2001 and 20xx, respectively. These results suggest that, all else being equal, the probability-of-occurrence of moist deep convection in the Black soil zone was lower in 1976 than in the base year 2001, and it will be higher in 20xx when the annual crop area reaches a maximum. The trend to continuous cropping had less impact in the drier Dark Brown and Brown soil zones.
Highlights
Summerfallow is cropland that is purposely kept out of production during a growing season.Traditionally farmers used this practice as a risk management strategy to improve their chances of growing a crop the following year
The adjustment factor was based on the difference between the monthly average albedo at Lethbridge over grass, and the monthly average albedo for annual crops, perennial forage and summerfallow determined from Moderate Resolution Imaging Spectroradiometer (MODIS) satellite image analysis from 2000 to 2003 for the province of Saskatchewan (Wang S., unpublished data)
−1.19 ± 0.24, and −1.24 ± 0.29, respectively in the Brown soil zone. These results indicate that in all soil zones, during the foliar expansion and reproductive period for annual crops, the probability-of-occurrence of moist deep convection, and its intensity if it occurred, would be the lowest in 1976, and the highest in 20xx when all of the potential agricultural areas have been seeded to annual crops
Summary
Summerfallow is cropland that is purposely kept out of production during a growing season. While upward sensible heat flux (QH) determines the depth of the CBL [16] and modifies its temperature profile, upward latent heat flux (QE), i.e., evapotranspiration (ET), moistens the CBL Both increase the convective available potential energy (CAPE), if conditions above the CBL remain unchanged. It was concluded that the moisture flux from the Prairie agro-ecosystem translates directly into changes in CBL moisture, which subsequently affects the magnitude of the potential energy available for moist deep convection, and the seasonal pattern of tornado days. Mean mid-growing-season Bowen ratios, mean solar noon global radiation values and Segal et al.’s [23] relationship were used to estimate the reduction in the LI, indicative of an increase in CAPE, associated with the land-use weighted ET in each study year
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