Geoscience Reference
In-Depth Information
When compared with statewide change areas, the near-decadal change (1992-
2001) occurring in the previously indicated subwatersheds, accounted for 80% of
the statewide change, indicating a recurrence of land conversion in these subwa-
tersheds. It is reasonable to expect that land cover change tends to happen near
previous areas of land cover change and this has been shown in other studies.
Change or urbanization begets more change (urbanization) which is important when
planning for meeting clean water requirements at local, state, and national levels
for environmental managers as it relates to policy implications and infrastructure
implementation.
To examine the aggregation level of this new imperviousness, a region cre-
ation approach was used by applying a “queen contiguity” (8 neighbors) to the
retrofit product (1992-2001) and the NLCD05 (2001-2005), followed by the bound-
ary clean operation. Zonal statistics were calculated for the regions and regions
attributed with their
majority
source class. The relative accuracy of the generaliza-
tion method used can be assessed by the attribution of developed land to spurious
classes, such as water, wetland, developed, etc., even though the amount of “con-
taminants” was minor. Forest and agriculture were the main contributors to the
developed areas, with an apparent increasing contribution from the former, as indi-
cated by their conversion ratios for each time period (1992-2001 and 2001-2005),
1.8 and 1.6 respectively. Total daily conversion rates appeared to double from
2.6 ha/day (1992-2001) to 4.9 ha/day (2001-2005), with agriculture and forestry
also doubling from their daily rates of 0.9 and 1.6 ha/day, respectively, from the
first to the second period. The contributions from all other land cover classes were
negligible. For control purposes, the spatial overlap between the two time period
generalizations was 107 ha - or 0.6% of the combined 1992-2001 and 2001-2005
change zones (excluding overlap).
8.3.3 Recurrent Addition of New, Compact, Impervious Areas
in Subwatersheds
The distribution of 2001 baseline TIA (pooled method) relative to total area for
those subwatersheds impacted by developed class change was assessed next. Based
on Schueler's (
1994
) three major critical watershed imperviousness intervals (i.e.,
0-10%, >10-25%, and >25-100%) it was found that nearly 14% of the approxi-
mately three million hectares (ha) in the study area fell in the impacted or threatened
watershed categories. When compared with the 2001 land cover, the majority of
the small relative increase in developed land observed four years later was repre-
sented by substantial growth in the low and medium intensity development classes
(Table
8.4
).
Using the land cover to calculate imperviousness conversion factors previously
described, a
net
increase in TIA of 3,392 ha, resulted for the whole study area
between 2001 and 2005. This represented an overall increase in TIA from 2.5-
2.7% for the selected subwatersheds, highlighting the importance of both local
concentration of imperviousness and its initial values (Figs.
8.9
and
8.10
).
