Geology topics

M.D. Tomer

Publications and source records attributed to M.D. Tomer.

4 recordsLinked to original sources

USGS2016

Effects of hydrology, watershed size, and agricultural practices on sediment yields in two river basins in Iowa and Mississippi

The specific sediment yield (SSY) from watersheds is the result of the balance between natural, scale-dependent erosion and deposition processes, but can be greatly altered by human activities. In general, the SSY decreases along the course of a river as sediments are trapped in alluvial plains and other sinks. However, this relation between SSY and basin area can actually be an increasing one when there is a predominance of channel erosion relative to hillslope erosion. The US Geological Survey (USGS) conducted a study of suspended sediment in the Iowa River basin (IRB), Iowa, and the Yazoo River basin (YRB), Mississippi, from 2006 through 2008. Within each river basin, the SSY from four largely agricultural watersheds of various sizes (2.3 to 35,000 km 2 [0.9 to 13,513 mi 2 ]) was investigated. In the smallest watersheds, YRB sites had greater SSY compared to IRB sites due to higher rain erosivity, more erodible soils, more overland flow, and fluvial geomorphological differences. Watersheds in the YRB showed a steady decrease in SSY with increasing drainage basin area, whereas in the IRB, the maximum SSY occurred at the 30 to 500 km 2 (11.6 to 193 mi 2 ) scale. Subsurface tile drainage and limits to channel downcutting restrict the upstream migration of sediment sources in the IRB. Nevertheless, by comparing the SSY-basin size scaling relationships with estimated rates of field erosion under conservation and conventional tillage treatments reported in previous literature, we show evidence that the SSY-basin size relationship in both the IRB and YRB remain impacted by historical erosion rates that occurred prior to conservation efforts.

Iowa, Mississippi↗

USGS2009

A simple approach to distinguish land-use and climate-change effects on watershed hydrology

Impacts of climate change on watershed hydrology are subtle compared to cycles of drought and surplus precipitation (PPT), and difficult to separate from effects of land-use change. In the US Midwest, increasing baseflow has been more attributed to increased annual cropping than climate change. The agricultural changes have led to increased fertilizer use and nutrient losses, contributing to Gulf of Mexico hypoxia. In a 25-yr, small-watershed experiment in Iowa, when annual hydrologic budgets were accrued between droughts, a coupled water-energy budget (ecohydrologic) analysis showed effects of tillage and climate on hydrology could be distinguished. The fraction of PPT discharged increased with conservation tillage and time. However, unsatisfied evaporative demand (PET - Hargreaves method) increased under conservation tillage, but decreased with time. A conceptual model was developed and a similar analysis conducted on long-term (>1920s) records from four large, agricultural Midwest watersheds underlain by fine-grained tills. At least three of four watersheds showed decreases in PET, and increases in PPT, discharge, baseflow and PPT:PET ratios (p < 0.10). An analysis of covariance showed the fraction of precipitation discharged increased, while unsatisfied evaporative demand decreased with time among the four watersheds (p < 0.001). Within watersheds, agricultural changes were associated with ecohydrologic shifts that affected timing and significance, but not direction, of these trends. Thus, an ecohydrologic concept derived from small-watershed research, when regionally applied, suggests climate change has increased discharge from Midwest watersheds, especially since the 1970s. By inference, climate change has increased susceptibility of nutrients to water transport, exacerbating Gulf of Mexico hypoxia.

Journal of Hydrology↗

USGS2007

Tree and stand transpiration in a Midwestern bur oak savanna after elm encroachment and restoration thinning

Oak savannas, once common in the Midwest, are now isolated remnants within agricultural landscapes. Savanna remnants are frequently encroached by invasive trees to become woodlands. Thinning and prescribed burning can restore savanna structure, but the ecohydrological effects of managing these remnants are poorly understood. In this study, we measured sap flow (Js) to quantify transpiration in an Iowa bur oak (Quercus macrocarpa) savanna woodland encroached by elms (Ulmus americana), and in an adjacent restored savanna after thinning to remove elms, during summer 2004. Savanna oaks had greater mean daily Js (35.9 L dm-2 day-1) than woodland oaks (20.7 L dm-2 day-1) and elms (12.4 L dm-2 day-1). The response of Js to vapor pressure deficit (D) was unexpectedly weak, although oaks in both stands showed negative correlation between daily Js and D for D > 0.4 kPa. An earlier daily peak in Js in the elm trees showed a possible advantage for water uptake. As anticipated, the woodland's stand transpiration was greater (1.23 mm day-1) than the savanna's (0.35 mm day-1), yet the savanna achieved 30% of the woodland's transpiration with only 11% of its sapwood area. The difference in transpiration influenced water table depths, which were 2 m in the savanna and 6.5 m in the woodland. Regionally, row-crop agriculture has increased groundwater recharge and raised water tables, providing surplus water that perhaps facilitated elm encroachment. This has implications for restoration of savanna remnants. If achieving a savanna ecohydrology is an aim of restoration, then restoration strategies may require buffers, or targeting of large or hydrologically isolated remnants. ?? 2007.

Forest Ecology and Management↗

USGS2007

Hydrogeologic controls on nitrate transport in a small agricultural catchment, Iowa

Effects of subsurface deposits on nitrate loss in stream riparian zones are recognized, but little attention has been focused on similar processes occurring in upland agricultural settings. In this paper, we evaluated hydrogeologic controls on nitrate transport processes occurring in a small 7.6 ha Iowa catchment. Subsurface deposits in the catchment consisted of upland areas of loess overlying weathered pre-Illinoian till, drained by two ephemeral drainageways that consisted of Holocene-age silty and organic rich alluvium. Water tables in upland areas fluctuated more than 4 m per year compared to less than 0.3 m in the drainageway. Water quality patterns showed a distinct spatial pattern, with groundwater in the drainageways having lower nitrate concentrations (<0.5 mg L-1 compared to upland areas (>10 mg L-1) as wells as lower pH, dissolved oxygen and redox, and higher ammonium and dissolved organic carbon levels. Several lines of evidence suggested that conditions are conducive for denitrification of groundwater flowing from uplands through the drainageways. Field-measured nitrate decay rates in the drainageways (???0.02 day-1) were consistent with other laboratory studies and regional patterns. Results from MODFLOW and MT3DMS simulations indicated that soils in the ephemeral drainageways could process all upland groundwater nitrate flowing through them. However, model-simulated tile drainage increased both water flux and nitrate loss from the upland catchment. Study results suggest that ephemeral drainageways can provide a natural nitrate treatment system in our upland glaciated catchments, offering management opportunities to reduce nitrate delivery to streams. Copyright 2007 by the American Geophysical Union.

Journal of Geophysical Research G: Biogeosciences↗