A Raft-Based Method for Lagrangian Sampling in Headwater Agricultural Watersheds

In agricultural landscapes, differences in management practices, soil characteristics, and topography can all interact to influence the delivery of water, sediment, and nutrients from farm fields to receiving ditches and streams. While a wide variety of conservation practice options are available to landowners and conservation professionals, it can be difficult to know where they are most cost effective because of a disconnect in information between plot-scale studies and monitoring data collected at watershed outlets. With advances in in-situ water quality sensor technology it is now possible to fill this knowledge gap.

We are employing a Lagrangian sampling technique to monitor a water parcel as it travels down an agricultural ditch network. With this approach, the behavior of small scale watersheds can be characterized more precisely, and important sites of new inputs and/or transformations can be identified. Our sampling approach is based on a small inflatable kayak that is capable of navigating narrow, shallow agricultural ditches. From the kayak, we deploy a sensor platform consisting of an optical nitrate sensor and a multiparameter sonde coupled with a GPS receiver which allows us to generate data sets of key parameters through a ditch and stream network with a spatial resolution on the order of 1 m. The platform allows for sampling in areas inaccessible to researchers by other means and by repeatedly paddling the same stream reach it is possible to identify and quantify in-stream nutrient dynamics at a range of spatio-temporal scales.

Preliminary results have been helpful for identifying key micro-watershed tributaries as important sources of nitrate, and assessing the role of wetlands in mediating nitrate loads. Long-term studies in two agricultural watersheds show the capability of the system to detect changes in nitrate concentration along flow paths in response to certain watershed features. Nutrient inputs from sub-catchments defined by tile ditch networks can be spatially resolved as well as impacts from wetlands and larger confluences. Repeated sampling over spring thawing and rain events shows runoff and tile loading impacts on in-stream nitrate concentrations in relation to meteorological events. Specific areas of interest to water conservation efforts were also sampled including sub-catchments containing conservation practices such as filter strips and cover crops. 

Ongoing efforts are focused on combining Lagrangian sampling with Eulerian monitoring at selected sites in order to identify biogeochemically important hot-spots and hot-moments by combining spatial datasets with water budgeting approximations. Further efforts using the platform seek to quantify landscape-specific loading mechanisms and the potential for in-stream processing of nutrients under certain biogeochemical conditions. Potential applications of this methodology include evaluation of the impact of specific practices such as saturated buffers, bioreactors, and constructed wetlands on water quality as well as identifying areas of concern within small watersheds.