Microtopography and Crop Vigor Changes Assessment Using Time Series of UAS Derived Data.

Deriving information from remotely sensed data is a critical component of food security and soil conservation efforts; however, obtaining high temporal and spatial resolution data can be expensive and time consuming when performing long-term small-scale process investigation studies. The use of small unmanned aerial systems (sUAS) with a consumer grade RGB camera to collect aerial imagery provides a cost-effective means for gathering frequent, highly detailed information. We utilized sUAS to monitor agricultural fields in Raleigh, NC, resulting in over 20 surveys from 2015-2018. A time series of high resolution orthophotos (3cm/pix) and DSMs (15cm/pix) were created using Structure from Motion (SfM) photogrammetry. The time series was processed and analyzed using an open source GRASS GIS temporal framework to investigate two phenomena: in-field variability of crop vigor and the evolution of ephemeral gullies. We used 3D crop height models to identify areas of high or low relative crop growth over four years of agricultural production. The impact of soil moisture on crop vigor was investigated by modeling overland water flow and solar radiation. Preliminary results show spatial coincidence of low flow accumulation, high solar radiation, and low relative crop heights. Furthermore, we observed two ephemeral gullies in the study area. The smaller gully was planted and covered in vegetation every growing season, but the larger gully was not vegetated for part of the time series. We analyzed the impact of agricultural activity and vegetation on erosion-deposition patterns and topographic evolution. Two aerial lidar surveys (2013 and 2015) were used for comparison. Over the four years covered by our sUAS surveys, the location and extent of these gullies did not change significantly despite multiple storms and crop rotations. The shape of the larger gully did evolve, however, with differences between the initial and current state reaching 0.5 m.