As agricultural regions are threatened by climate change, warming of high latitude regions and increasing food demands may lead to northward expansion of global agriculture. While socio-economic demands and edaphic conditions may govern the expansion, climate is a key limiting factor. Extant literature on future crop projections considers established agricultural regions and is mainly temperature based. We employed growing degree days (GDD), as the physiological link between temperature and crop growth, to assess the global northward shift of agricultural climate zones under 21st-century climate change. Using ClimGen scenarios for seven global climate models (GCMs), based on greenhouse gas (GHG) emissions and transient GHGs, we delineated the future extent of GDD areas, feasible for small cereals, and assessed the projected changes in rainfall and potential evapotranspiration. By 2099, roughly 76% (55% to 89%) of the boreal region might reach crop feasible GDD conditions, compared to the current 32%. The leading edge of the feasible GDD will shift northwards up to 1200 km by 2099 while the altitudinal shift remains marginal. However, most of the newly gained areas are associated with highly seasonal and monthly variations in climatic water balances, a critical component of any future land-use and management decisions.
Abstract:The spatial variability of soil water content can be measured with the ground wave velocity of ground-penetrating radar (GPR) using short antenna offsets, but picking the correct ground wave arrival time is rather difficult. In applying the GPR ground wave method to soil water content estimation it is also important to know the effective sampling depth of the method. Uniform drainage experiments were conducted with 100 and 450 MHz GPR antennas using 1Ð0 and 2Ð0 m fixed antenna separations on a sandy loam soil to investigate time zero picking methodologies and to estimate the sampling depth of the GPR method. The GPR water content data were compared with time-domain reflectometry (TDR)-measured data using six vertical TDR probes of different lengths. Time zero was calculated from an air calibration at a 2Ð0 m antenna separation and from wide-angle reflection and refraction data, and a difference was found between the two time-zero calibration methods. A method was analysed to determine the arrival time of the leading edge of the direct ground wavelet using the arrival time of the peak amplitude, since the arrival time of the leading edge of the ground wave can be difficult to pick. Regression analysis showed that the GPR (100 MHz) measured water content was not different from the water content measured with TDR at 0-0Ð1 m depth, implying that this may be a reasonable estimate of the GPR ground wave method's sampling depth. A similar analysis based on the differences between the 0-0Ð2 m TDR and the GPR shows that the effective sampling depth of the direct ground wave of the 450 MHz data is less than the sampling depth of the 100 MHz data.
et al., 2003a;Parkin et al., 2000;Rucker and Ferré , 2002). The direct GW method, the The direct ground wave method of ground penetrating radar (GPR) subject of this paper, may be suitable for cost-effective has been suggested as a cost-effective means of estimating field-scale soil moisture variability for irrigation and water resource manage-measurement of soil moisture variability at the field ment. Knowing the sampling depth of the GPR direct ground wave scale, but more information on sampling depth is needed. (GW) is very important because it is critical to know the depth whenThe main reason that GPR methods of measuring measuring soil moisture in the field. Few studies have addressed this soil moisture are of interest is that they provide a means particular aspect of the GPR method. Numerical simulation of GPR to monitor large areas and soil volumes relatively electromagnetic waves using GPRMAX2D was performed for twoquickly. The methods listed above typically sample vollayer soil models to estimate the direct GW sampling depth for soil umes much less than 1 m 3 and, because they are intrumoisture. Dry over wet soil layers and wet over dry soil layers were sive, are best suited for monitoring at a single location modeled by using appropriate dielectric permittivity values for each vs. time. On the other hand, GPR equipment generally layer. Model runs were conducted for a gradually decreasing upper samples volumes of 1 m 3 or more and can be easily layer thickness. The GW sampling depth was estimated as the upper dry or wet layer thickness when the modeled GW velocity decreased moved from place to place since it is nonintrusive. or increased by 5% as affected by the lower wet or dry layer, respec-With all indirect soil moisture measurement techtively. It was found from this modeling exercise that the GW sampling niques it is important to understand their sampling voldepth changed with the antenna frequency as well as the moisture ume. With the GPR direct GW method, neither the content of the upper layer. A very strong linear relationship (r 2 ϭ effective sampling area nor sampling depth is easily 0.98) was found between the wavelength and the sampling depth of quantified. The sampling area can be roughly described the GPR direct GW.
The soil water content distribution at two field sites was measured with the air launched surface reflectivity method using a standard GPR system elevated 1 m above the surface. Time domain reflectometry (TDR) measurements of water content were also acquired at these sites. At one site, water was applied to the surface in two separate experiments and the water content was measured during drainage. At the other site, a water content profile was acquired across two terrain types, a flat grass field and a corn field. Although the GPR surface reflectivity method was able to map the water content distribution at both sites there were substantial
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