Date of Award


Degree Name

Master of Science



First Advisor

Williard, Karl


Critical Source Areas (CSAs) of phosphorus (P) are areas within a watershed that have a high propensity to export P to surface waters. CSAs contain two factors: source and transport factors. Source factors include soil P status and fertilizer and manure inputs, while transport factors include hydrologic and erosion processes that mobilize P. The aim of this study was to: 1) identify CSAs of P in an agricultural watershed and the stormflow dynamics controlling P export and 2) to delineate CSAs of P at the agricultural field scale using georeferenced soil test P (STP) and a digital elevation model (DEM) in a geographic information system (GIS). Soil test P (STP) along with dissolved reactive P (DRP), particulate P (PP), and total P (TP) in soil water, groundwater, and surface runoff were monitored in three small (< 8 ha) agricultural watersheds located in Decatur, Illinois, each situated within a separate experimental field. Further, volumetric water content (VWC) was continuously monitored on topographic positions, e.g. foot slopes, hill slopes, and shoulder slopes, to determine topographic position influence on soil moisture distribution. Repeated measures mixed models analysis showed that foot slopes (32.2%) had significantly higher VWC than hill slope (29.6%) and shoulder slopes (30.9%) during the growing season, while foot slopes (38.9%) and hill slopes (38.9%) had significantly higher VWC than shoulder slopes (34.9%) during the dormant season. Persistent shallow groundwater tables were implicated to control spatial and temporal VWC moisture distribution. Both foot slopes and hill slopes were implicated as transport areas. Repeated measures mixed models analysis also showed that foot slopes (73 kg ha&minus1) had significantly higher STP than hill slopes (28.9 kg ha&minus1) and shoulder slopes (33.8 kg ha&minus1) most likely due to the erosion and deposition of sediment from upper slopes to lower slopes. Foot slopes were consequently classified as source areas. A surface runoff event revealed near stream saturation and flushing of soil moisture from upper slopes to lower slopes, indicating that the watersheds are variable source area driven. The peak of PP on the rising limb of the hydrograph was attributed to near stream sediment mobility while the peak of DRP on the falling limb was attributed to flushing of upper slope soil moisture via subsurface flow. GIS delineation of CSAs at the agricultural field scale was conducted to pinpoint precise locations within a field to implement precision P management. The topographic position index (TPI) along with a modified version of the slope classification model &mdash both of which were created by Weiss (2001) and automated by Jenness (2006) &mdash were used to delineate foot slopes, hill slopes, shoulder slopes, and flat areas within a 91.2 ha agricultural field from a DEM. Transport factors were, again, identified as foot slopes and hill slopes. Further, georeferenced STP data collected in spring 2010, fall 2010, and fall 2011 were averaged and interpolated using ordinary kriging to generate a single surface that represented three year spatial soil P status within the agricultural field. Source factors were identified as areas in the field that were excessive in soil P for corn-soybean production. A CSA model was created that identified areas where both source factors and transport factors overlapped. CSAs of P occurred on 2.3 ha of the agricultural field and occurred near grass waterways and roadside drainage ditches. A one way analysis of variance (ANOVA) along with a Tukey mean separation procedure of soil P on the four topographic positions was used to characterize soil P spatial dependencies on landscape attributes associated with topographic position. Foot slopes (79.5 kg ha&minus1) and flat areas (92.9 kg ha&minus1) had significantly greater soil P than hill slopes (59.8 kg ha&minus1) and shoulder slopes (49.8 kg ha&minus1) due to depositional and sink attributes. Depositional attributes exhibit concave curvature, e.g. foot slopes. This curvature effectively reduces the velocity of surface runoff so that sediment bound P suspended in surface runoff can be deposited on the soil surface. Sink areas accrue P inputs but do not lose P to erosion via surface runoff. These areas exhibit linear, non-sloping planes, e.g. flat areas, that are not conducive to surface runoff. Although topographic position explains the spatial dependencies of source and transport factors, the CSA model was able to pinpoint where CSAs of P spatially occur within the agricultural field which can allow for precision P management.




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