Monitoring and Modeling the Effect of Agricultural Drainage and Recent Channel Incision on Adjacent Groundwater-Dependent Ecosystems

Philip J Gerla

doi:10.3390/w11040863

Abstract

Channel incision isolates flood plains, disrupts sediment transport, and degrades riparian ecology. Reactivation and periodicity of incision may affect the water table and hydrological conditions far beyond the stream margin. Long-term incision and its recent acceleration along Iron Springs Creek, North Dakota, USA, has affected adjacent ecosystems. An agricultural surface drain empties directly into the original spring-fed source of the creek, which triggered channel erosion both up- and downstream. Historical maps, recent LiDAR, and field surveying were used to characterize incision since ditch excavation in 1911. Although the soils are sandy, small hydrological gradients impede natural drainage in the surrounding stabilized dunes. Incision resulting from expanded drainage and increased precipitation has been as much as 5 m. Numerical models of lateral groundwater profiles corroborated with field measurements show that the nearby water table responds quickly, becoming deeper and less variable. With 1 m of recent incision, model evapotranspiration rates are decreased 50% to 15% from the channel margin to 1 km, respectively, and the hydropattern disrupted >1 km. Species diversity is reduced and floristic quality is 25% less near the drain. A near-channel solution to erosion—fencing out cattle—failed to mitigate the problem because a broader watershed approach was necessary.

Highlights

Stream channel incision occurs naturally and under conditions of human-induced land cover change, including excavated agricultural drains that essentially constitute “controlled” incision.Channel deepening can occur as a result of many factors, including climate change [1,2], lowered stream base level [3], flashier stream flow [4], channelization [5], and changes in sediment load [5,6,7]
Objectives of this study include: (1) show how groundwater modeling can portray hydrological conditions that have been disrupted by drainage and channel monitoring and modeling can portray hydrological conditions that have been disrupted by drainage incision, (2) describe how these disturbances alter plant communities at distances far beyond those and channel incision, (2) describe how these disturbances alter plant communities at distances far generally considered to be the channel’s riparian zone, and (3) outline methods to forecast the extent to beyond those generally considered to be the channel's riparian zone, and (3) outline methods to which the seasonally shallow water table and accompanying evapotranspiration may be altered within forecast the extent to which the seasonally shallow water table and accompanying the watershed if headward erosion remains unchecked
The results show that uncontrolled incision on Drain 10 has led to the obvious erosion and destruction of the natural seeps at the head of Iron Springs Creek, but to a lower water table and loss of wetlands

Summary

Introduction

Stream channel incision occurs naturally and under conditions of human-induced land cover change, including excavated agricultural drains that essentially constitute “controlled” incision.Channel deepening can occur as a result of many factors, including climate change [1,2], lowered stream base level [3], flashier stream flow (often following human development and greater runoff) [4], channelization [5], and changes in sediment load [5,6,7]. Incision triggered by land cover changes can occur rapidly, with unintended consequences, which include the channel being cut off from its flood plain, stream enlargement [8], severe erosion and deposition [9], and the destruction of riparian zones because of reduced flood water and lowering of the water table [10]. This last process can be profoundly deleterious to groundwater-dependent ecosystems [11]. The deleterious effects may go unnoticed or attributed to other unrelated processes because of landscape heterogeneity and a lack of monitoring and quantitative observation

Objectives

Methods

Findings

Discussion

Conclusion