However, according to Earl , the two concepts are not independent of each other, meaning if a soil is deemed workable, then it is also assumed trafficable. Since critical soil moisture thresholds for both trafficability and work ability are generally recognized to be just below field capacity , differences in PET and soil texture are expected to drive differences in time-to-trafficability, assuming no water additions have occurred. General decision support tools are needed to guide famers in their choices about Ag-MAR timing and effect on trafficability, given the complexity of soil hydrology and seasonal climate differences. A number of studies have used either θfc or available water capacity to help define trafficability or work ability soil moisture thresholds. For example, a nationwide trafficability assessment was done for Canadian Prairie soils identified as suitable for annual cropping, using the Versatile Soil Water Budget model of 0–5 cm soil survey texture data overlaid with a 10-km climatological grid . In their modeling effort, expert advice was cited in their definition of trafficability as 80% of θfc for clays and 90% of θfc for all other soil textures, using θfc estimates from Canadian soil survey. Similar soil moisture modeling approaches informed a number of Canadian studies in the 1970’s where either trafficability or work ability thresholds ranged from 90-to-99.5% of θfc, considering soil moisture in the upper 12–30 cm . In the mid-western US, a similar approach was used to estimate working days for spring tillage,square plant containers setting their “tractability” threshold to 92–100% of θfc in the upper 7.5 cm and 94–100% of θfc from 7.5 to 15 cm depth, requiring more depletion for finer textures, with model estimates validated for accuracy using reported field observations .

However, often missing in this research line are textural class specific rules for determining trafficability where the risks of compaction vary across classes. For example, Leenhardt and Lemaire developed a threshold of AWC to define suitable days for sowing crops in southwest France by calibration to operational records from one experimental farm, using 70% of AWC in the upper 10 cm soil from mid March to mid-May and a less conservative 80% of AWC for dates later than mid-May, but these definitions were equally applied across all textures in the study area. The same assumptions were employed by Maton et al. in a maize production simulation study where simulated sowing dates corresponded fairly well with sowing dates reported by farmers, with 19% of actual field work days reported during periods when the model estimated the soil to be too wet. In Norway, farmers are reported to view soils as generally workable for the purpose of seedbed preparation and cereal sowing in the range of 85-to-95% of θfc . This informed a climate impact assessment of work ability on Norwegian cereal production, where the researchers used 85% of θfcin the upper 20 cm as the threshold for defining a working day in their soil moisture modeling approach. Other researchers have found a stronger relationship to farm reported days available for field work when the moisture threshold is based on soil consistence data compared to θfc . The Atterberg limit tests were originally developed as soil engineering tests of soil consistence to identify water contents at which a soil’s behavior changes: the liquid limit represents the critical water content at which a soil will flow like a liquid when jarred and the plastic limit represents the critical water content at which a soil transitions from a crumbly, semi-solid to a moldable material that is more susceptible to compaction.

In a detailed study of soil moisture retention and compaction of three different soil textures, Mapfumo and Chanasyk concluded that fine textured soils with higher plasticity indices were at greater risk of compaction, because their plastic limits were below θfc. Thus, Atterberg limits offer a means by which to logically scale, based on soil compaction risk, the aforementioned trafficability thresholds of θfc. In this study, the overall objective was to estimate typical time-to trafficability after Ag-MAR for a wide variety of soils and climates encountered in California. Time-to trafficability in this study is defined as the number of days without precipitation to reach a surface soil moisture content necessary to support typical agricultural vehicle traffic and/or conducive to shallow secondary tillage or planting. Many agricultural operations occurring during the period of available water for Ag-MAR require either surface contact or shallow soil contact < 10 cm deep . To do this, soil texture specific moisture thresholds were defined. Specifically, the mean plasticity index by textural class, assumed to be an indicator of compaction risk, was derived from the Soil Survey Geographic database and then rescaled to thresholds of θfc ranging from 85% of θfc for textures with the highest plasticity index to 95% of θfc for textures with the lowest plasticity index. Moreover, because it is widely recognized that it is challenging to objectively define θfc , θfc was also estimated for each soil horizon as the water content when drainage becomes negligible . HYDRUS-1D was then used to simulate inundation of soils for Ag-MAR by applying 30 cm H2O day -1 for four days, followed by drainage and evaporative drying across a range of soils, climate, and times of year to develop temporally and spatially explicit rain-free time-to-trafficability estimates. Rain-free time-to-trafficability estimates following Ag-MAR, derived from H1D simulation of soil moisture drydown, revealed clear differences across soil textural classes and time of year for a given location.

These results are unique in that they provide specific, textural class and PET based rain-free time-to-trafficability estimates that can help guide agricultural operation timing decisions following deep soil wetting. In contrast, other studies have estimated the number of trafficable or workable days per month or year for specific locations, such as in Canada and the United Kingdom , to guide identification of suitable agricultural lands, to appropriately size agricultural equipment by region, or to assess possible climate change impacts on trafficability or work ability. Other studies have also estimated suitable days for planting crops as an input to larger agricultural production models, such as in France and Norway , but not presented trafficability results in detail, relative to differences in soil texture or weather, that form the basis of these suitability predictions. Seasonal effects on rain-free time-to-trafficability were especially pronounced in the more sensitive loams and silt loams.Specifically, median time-to-trafficability for the clay texture class approached an asymptote around 13 rain-free days as PET increased from 3-to-6 mm day -1 , whereas time-to-trafficability continued to decrease steadily in most other textures from 3-to-6 mm PET day -1 . Interestingly,plastsic pots manufacturers at the coolest end of the modeled spectrum, loams and silt loams had the longest median time-to-trafficability, approaching 40 rain-free days. However, at this cooler end of the PET spectrum , time-to-trafficability changed most drastically with respect to PET and was also relatively uncertain within all textural classes, as indicated by wide interquartile ranges . This amplification of differences across soils and climate at relatively low PET is because generally smaller evaporative fluxes add up to larger temporal differences across simulations. By 4 mm PET day -1 , the time-to-trafficability estimates by textural class converged into five apparent groups, ordered from least to greatest: sands, loamy sands, sandy loams, and sandy clay loams; clay loams, silty clay loams, and loams; silt loams; silty clays; and clays . When mapped, median estimates for rain-free time-to-trafficability exceeded 25 days for a typical January across 36% of the study area . This suggests that Ag-MAR practiced in January will all but eliminate opportunities for subsequent agricultural traffic without risk of compaction in this region, which is especially relevant to almond growers whose trees start blooming in mid-February. On the other hand, this region is composed of finer textured soils , which are recognized as less suitable for Ag-MAR due to slower water percolation and longer root zone moisture residence time, which may be a risk to the health of many perennial crops .

A month later in February, rain-free time-to-trafficability exceeded 20 days across only 7% of the study area . By March, time-to-trafficability estimates were less than 10 rain-free days for 52% of the study area and for 82% of the study area in April . In presenting specific estimates of time-to-trafficability that are dependent on both climate and soil hydrology, these estimates are unique but also sensitive to assumptions used to define the soil moisture trafficability threshold. Rotz and Harrigan performed a sensitivity analysis of their whole farm simulation model and concluded that the predicted number of available days for field work were most sensitive to the trafficability threshold. The results presented here could benefit from additional research to fine-tune trafficability moisture thresholds and test the associated methodology. This warrants some discussion to explain how time-to-trafficability estimates presented here should be interpreted as indicating waiting times to low risk of soil compaction, especially for textural classes with lower plasticity indices. While θfc has been used and validated widely as a critical threshold to define trafficability , the concept of θfc does not actually reflect any definitive soil moisture status, because soils continue to drain indefinitely. Rather, θfc is intended to reflect the soil moisture status when drainage becomes relatively negligible. In field-based methods to determine θfc, this state of negligible flux is commonly assumed to take place 48 h after wetting and tarping the surface of a study soil, while warning practitioners that “the hydraulic conductivity of coarser-textured soils may become “negligible” in < 24 h, while some finer-textured soils may continue to drain at a “non-negligible” rate for periods exceeding 1 week or more” . Somewhat surprisingly, H1D simulations in our study, which also included modeling of evaporative moisture loss, suggested that even coarse textured surface horizons take more than 5 days to reach the assigned θfc when PET is relatively low, such as in February, but the time was sensitive to climate as demonstrated by the faster arrival to θfc in April . Moreover, further examination revealed that the drainage flux at 10-cm depth reversed and became an upward flux before θfc was reached across all texture classes. This showed that capillary rise and evaporation, as opposed to drainage, were the ultimate processes determining when each surface horizon dried to its defined state of θfc in H1D. θfc estimates were derived from each soil’s moisture retention parameters based on a detailed modeling study relating these parameters to soil moisture status when the drainage flux became “negligible” . In their work, they defined “negligible” in a comparative analysis, by defining θfc when the drainage flux reached 0.001, 0.01, and 0.1 cm day -1 . They also analyzed the effect of the simulated profile length, comparing 1-, 10-, and 100-cm profiles and found that, while the derived θfc was not dependent on profile length, the time to arrive at that derivation was very sensitive to profile length. In the case of 100-cm profiles, time to arrive at a “negligible” drainage flux took 100 days or longer, so they settled on a 1-cm profile for the remainder of their study to save computational resources. But what are the implications of this derivation method for a deeper profile that has been saturated? Emulating the work by Twarakavi et al. by setting PET equal to 0 to make evaporation negligible, the soil water content was identified when the drainage flux at various depths becomes “negligible” in a typical 200-cm sandy loam profile. Considering only the upper 30 cm of this profile, the time it took to reach a negligible drainage flux increased steadily from 2.2 days at 1-cm depth to 58.4 days at 30-cm depth . Likewise, water contents corresponding to these “negligible” fluxes decreased steadily from 0.261 to 0.208 from 1-cm to 30- cm depth in this 200-cm profile. In the drainage-only simulation, only by reducing the total profile length was it possible to observe a soil water content that approached the θfc assigned to this soil by Eq. 7 in Twarakavi et al. within a reasonable time frame .