Three overarching questions were asked at the beginning of this work:
• Can a water and energy balance be performed for the IVS and is senescence able to be seen in a water balance?
• Does dew influence the radar backscatter values and is this influence significant?
• Can the dew influence on the backscatter be modeled and how is dew incorporated in the model?
Each question was investigated using data collected over the same time period over the IVS.
With the analysis, each question can now be answered. An energy balance was performed on the IVS and the results indicate that it was successful in describing the net radiation’s partitioning to the different fluxes. Before this conclusion was reached, the energy budget had to be closed via a combination of accounting for the soil heat storage and the residual radiation from the measurements. After the budget was forced closed, it gave confidence that the energy budget within the IVS can be accurately described and the latent heat values could be used to estimate the evaporation from the IVS. The evaporation within the field was the only variable within the water balance that was not measured directly as it had to be estimated from the latent heat which was closed using the Bowen ratio. The soil moisture and precipitation were measured directly. When the water balance was preformed, it was determined that there was an average of 1.59 mm of storage within the field. Some of the too much unaccounted for water could have been runoff which was assumed to be zero. Other possibilities that may account for some of this water may be from errors associated with the measuring of the precipitation within the field and with the EC towers in measuring the surface fluxes.
The water balance showed that there was an increase in the amount of water storage within the IVS over the three day period. Because of the possible increase in storage in the field over the three days, it would mask any exiting of water that is not captured within the evaporation and soil moisture storage term. The senescence reported over the three day period was 1.4 kg/m2 (equivalent to mm) from DOY266 to DOY268. The intercepted precipitation and dew dry-off are assumed to be captured in the evaporation term via the latent heat. Yes, a water and energy balance can be conducted over the IVS. Were both able to balance? No. The assumptions of no run off or drainage and all the precipitation infiltrated the soil and the combination of point and field averaged measurements used together may have contributed to this fact.
Dew and intercepted precipitation influence the radar backscatter as measured by the PALS overpasses. Both DOY267 and DOY268 had a wet canopy for some of the overpasses. Both times, the overpasses that coincided with the leaf wetness sensors indicating a wet canopy, the radar backscatter was significantly different than when the canopy was designated dry.
However, dew and intercepted precipitation had different effects upon the radar backscatter depending upon the polarization of interest. Intercepted precipitation both increased and decreased the observed radar backscatter, causing an increase in the backscatter for HH and HV, but a decrease for VV-polarization. On DOY267, both R8 and R10 overpasses were during the wet canopy time period and R12 was over a dry canopy. Looking at Table 2.3 and Figure 2.10, the radar backscatter values increased for VV-polarization while for HH- and HV-polarization, it decreased as the canopy dried.
Dew also has a polarization dependence on the impact it makes to the radar backscat-ter measurements. The polarization affects the magnitude of the change, not whether the backscatter increases or decreases. For all polarizations, dew increased the observed backscat-ter significantly. Pass R14 was down while the canopy was still wet with dew though in the drying phase of the dew. R15 was taken well after the canopy was indicated to be dry based off the leaf wetness sensors. For each polarization, the observed backscatter decreases for R15 from R14 (see Figure 2.12) while no other variable that affected the backscatter changed. The VV polarization means had the largest change from the R14 to R15 with HH and HV having
similar changes. This is opposite of dew’s effect on the passive measurement of soil moisture below a soybean canopy, which is an increase in brightness temperature (Hornbuckle, unpub-lished). An increase in brightness temperature indicates a possible decrease in soil moisture while an increase in the radar backscatter indicates a possible increase in soil moisture.
Dew was incorporated into the backscatter model by adding twice the amount of dew to the vegetative water content. The modeled values shifted closer to the observed values with this method but still under predicted the observed values. Increasing the water content to account for dew could detract from the model’s value as it shifts the dominating factor away from the soil and to the vegetation canopy. As noted in the introduction, there does exist a threshold above which the vegetation reduces the sensitivity to soil moisture changes and at a high enough water content, masks the soil completely. Seen in Figure 3.8, the soil moisture contribution to the backscatter decreases and slowly start leveling off but is lower than the contribution to the backscatter from the other terms. Anything larger than the doubling of the dew and adding to the vegetation water starts pushing the vegetation water content high enough that it will begin to decrease the soil moisture sensitivity. At the same time, assumptions about the relative impact of dew on the canopy can start becoming unreasonable.
Does dew and intercepted precipitation make an impact on the observed radar backscatter of soil through a soybean canopy? Yes. Is it possible to model the impact dew has on the bacskcatter? Yes, but it might not be as simple as doubling the dew amount and adding it to the vegetative water content. The data collected and presented here only had two passes over a soybean field during the drying phase of the dew event. This dew event was reported to be a fairly heavy dew as well. More measurements during the middle of the growing season when the vegetative water content is more stable could reduce some of the errors of the measurements and help show the dew effects. More measurements would also span more soil moisture conditions which would help show the sensitivity of the backscatter to changes in the soil moisture. More dew events and different dew amounts could also show if the impact on the backscatter is dependent upon the dew amount and what the right increase of the vegetative water content is to account for it. Measurements of the amount of intercepted precipitation would be used
to determine if the dew correction applies to that situation as well. Future measurements over different crop types apart from soybeans can help determine if the impact reported here is consistent across all row crops or if different crops are impacted differently.
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