Agriculture Reference
In-Depth Information
the Kalamazoo River watershed (Fig. 11.1) or just north of it in southern Barry
County. A rich body of KBS research on the limnology and ecology of local lakes
and streams that has been published since the 1970s aided in the interpretation of
patterns and processes.
What can be learned from patterns of water chemistry across landscapes?
By making simple measurements of specific conductance, we find that the total
ion content of water changes dramatically across the landscape, and the patterns
observed reflect biogeochemical processes along subsurface and surface flow paths
(Fig. 11.2). Ion content increases greatly as water from precipitation passes through
soils and the underlying unsaturated zone before reaching groundwater reservoirs.
Streams largely reflect the chemistry of their groundwater sources. In contrast,
lakes and particularly wetlands tend to have lower and more variable total ion con-
tents, and hence specific conductances, mainly due to variable relative contribu-
tions of groundwater and direct precipitation. These changes in water chemistry
have implications for the biological availability of nitrogen (N), phosphorus (P),
and silicon (Si)—the nutrients that often limit aquatic primary production. Total ion
content also affects mineral precipitation reactions—particularly those involving
calcium carbonate—that, in turn, affect the transport and fate of pollutants. In this
chapter, we evaluate landscape patterns in water quality, including both major ions
and key nutrient forms, and consider how the effects of agricultural activities are
superimposed on natural patterns and processes.
From Precipitation to Groundwater
Precipitation Amount and Variability
Southern Michigan's climate is continental and is significantly influenced by the
Great Lakes. Mean air temperatures at KBS range from -3.8°C in January to
22.9°C in July (1981-2010; NCDC 2013). Mean seasonal cycles of precipitation
and Thorthwaite potential evapotranspiration (PET) are typical for this part of
North America (Crum et al. 1990, Fig. 11.3). From 1951 to 1980, the mean annual
precipitation for KBS was 855 mm, and the difference between precipitation and
PET was 124 mm. And during the next 30 years (1981-2010), the mean annual pre-
cipitation increased 17% (1005 mm). Total precipitation varies considerably from
year to year; since 1996, water levels in a number of local wetlands and lakes near
KBS site have varied over 1-2 m (for an example, see Robertson and Hamilton
2015, Chapter 1 in this volume), reflecting this interannual climate variability.
Hydrogeology and Landscape Flow Paths
Southwest Michigan's recently glaciated landscape is distinct because of its high
degree of linkage between surface waters and groundwaters and the predomi-
nant influence of groundwater discharge on streams, rivers, most lakes, and many
wetlands (Grannemann et al. 2008). The landscape geomorphology is largely a
reflection of the most recent continental glaciation, when the Saginaw and Lake
