Using vegetation indices from satellite remote sensing to assess corn and soybean response to controlled tile drainage

Controlled tile drainage (CTD) is a management practice designed to retain water and nutrients in the field for crop use. CTD has shown promise for improving water quality and augmenting crop yields but findings are often restricted to field and plot scales. Remote sensing is one of the alternatives to evaluate crop responsiveness to CTD at large spatial scales. This study compared normalized and green normalized difference vegetation indices (NDVI and GNDVI) for corn (Zea mays L.) and soybean (Glycine max L.) among CTD and uncontrolled tile drainage (UCTD) fields in a ∼950 ha experimental watershed setting in Ontario, Canada from 2005 to 2008. The indices were derived from Landsat-5 and SPOT-4 satellite imagery. Log-transformed NDVI and GNDVI for soybean (R3–R6 growth stage) and corn (VT to R5–R6 growth stage) crops were higher significantly (p ≤ 0.05) for CTD, relative to UCTD for 50% (soybean) and 72% (corn) of both the log-transformed NDVI and GNDVI image acquisitions compared; only 17% and 13% were significant (p ≤ 0.05) in the reverse direction (UCTD > CTD). Log-transformed NDVI and GNDVI standard errors for CTD, relative to UCTD fields, were lower for 65% of the significant corn and 71% of the significant soybean NDVI and GNDVI comparisons for the growth stages noted above. This finding suggested overall more uniform crop growth for CTD fields relative to UCTD fields. Observed yields from a subset of commonly managed CTD and UCTD fields in the study area were not significantly different from each other (p > 0.05) with respect to tile drainage management practice; however, 87% of these paired yield comparisons indicated that CTD mean corn/soybean grain yields were greater than or equal to those for UCTD. On average, CTD observed corn and soybean grain yields were 3% and 4%, respectively, greater than those from UCTD. From observed yield and NDVI and GNDVI observations, vegetation indices vs. yield linear regression models were developed to predict grain yields over a broader land base in the experimental watershed area. Here, predicted mean yields were 0.1–11% higher for CTD corn and −5% to 4% higher for CTD soybean, relative to UCTD crops; but results varied between manured and non-manured fertilizer practices. Eighty-nine percent of the standard deviations for these yield predictions were lower for CTD relative to UCTD. The results of this study indicate that at a minimum, CTD did not adversely impact corn and soybean grain yields over the time span and field environments of the study, and based on the weight of evidence presented here, CTD shows general promise for augmenting crop performance. Finally, remote sensing derived vegetation indices such as NDVI and GNDVI can be used to assess the impact of agricultural drainage management practices on crop response and production properties.

Research highlightsThis study demonstrated that utilizing high spatial resolution imagery from multi-spectral remote sensing platforms to derive vegetation indices can be useful for determining the impact of controlled tile drainage on crop growth at large spatial scales. Such information can be especially valuable for crop growth modeling and yield forecasting. This study found: (i) at growth stages ≥V10 for corn and ≥R4 for soybean, a majority of the statistical comparisons conducted indicated that controlled tile drainage increases significantly NDVI and GNDVI for corn and soybean over the experimental study area for manured and non-manured fields, (ii) controlled tile drainage increases modestly the uniformity in vegetation indices of corn and soybean, relative to conventional tile drainage, (iii) there was no significant difference among observed controlled tile drainage and conventional tile drainage crop grain yields at the p = 0.05 level, yet 87% of the observed grain yield comparisons indicated field mean grain yields for controlled drainage were greater than or equal to those for conventional tile drainage (observed corn and soybean yields were 3% and 4% greater under controlled tile drainage practices), and (iv) using site-based grain yield vs. vegetation index regression models for specific crop growth stages to predict crop yield from vegetation index pixel information (NDVI and GNDVI) over a broader spatial extent in the experimental area, it was shown that predicted non-manured corn and soybean grain yields under controlled tile drainage were (7–11%) and (3–4%) higher relative to conventional tile drainage, respectively. These percentages for manured corn and soybean were (0.1–3%) and (−5% to −2%), respectively. Hence, although CTD effects on observed grain yield were insignificant from a statistical standpoint, the results here are promising in that crops were also not adversely impacted by CTD. This, coupled with the water quality benefits of CTD, suggests broadly that CTD is a practice that has multiple benefits to the producer and the environment.