irrigation channel provides year-round water to 6.5 ha of paddy fields. The river
within the lowland areas of the catchment is mainly fed by run-off, interflow and
baseflow from irrigated rice fields and fish ponds. From July to September, when
the buffer capacity in the reservoir is exceeded, reservoir spillover also feeds the
river. Due to their limited storage capacity, irrigation reservoirs can cause hazards
as a result of rainfall variability and irrigation management practices during the
monsoon season (see also Sect. 3.3). Particularly during the monsoon season, a
sequence of heavy rainfall events followed by a typhoon can trigger floods, as water
release cannot be controlled once spillover is activated. This study monitored
discharges released from the reservoir and at the sub-catchment outlet during the
wet season of 2008, and also analyzed total organic C and total N content (i.e.,
particulate and dissolved fraction) during several rainfall events which produced a
wide range of rainfall intensities and magnitudes (Schmitter et al. 2012).
3.4.2 Importance of Reservoir Management and Its Impact
on Carbon and Nitrogen Redistribution
During our study, in the absence of rainfall, the water in the reservoir contained on
average 4.7 Æ 1.2 mg L
À1 organic C and 3.8 Æ 1.6 mg L
À1 total N. The low C/N
ratios observed in the reservoir (Æ2) were most likely the result of in situ production
of C and N during decomposition, as aquatic plants are much lower in C when
compared to terrestrial plants (Beusen et al. 2005) leading also to an accumulation
of mineral N, such as NH 4
+ . However, during rainfall events, measured
concentrations at the outlet of the sub-catchment rose rapidly, and values up to
311.4 mg L
À1 organic C and 52.6 mg L
À1 total N were found. The increase in such
concentrations during rainfall events showed a high variability (Fig. 3.10), and a
combined analysis of the pollutograph and hydrograph profiles at both gauges
indicated the presence of non-point sediment sources along the irrigation channel,
which increased loads in the irrigation water. This was also reflected in changes to
the organic C/N ratios at the outlet, which increased during rainfall events to a value
of 7, as compared to a value of 2 for water from the reservoir.
Fluctuations in organic C and N concentrations in the irrigation channel were
highly dependent on the intensity and magnitude of rainfall, the presence of
antecedent dry weather periods and on the amount of irrigation water released
from the reservoir – including as a result of operational management decisions
made during events (i.e., changes in water regimes). A decrease in water released
from the reservoir during rainfall events did result in higher C and N concentrations
in the irrigation channel when compared to similar events when more water was
released from the reservoir.
A quantification of the several contributory water sources (i.e., reservoir and
overland run-off) and their water quality allowed for the estimation of nutrient loads
irrigated to the paddy fields. On the basis of the estimated irrigation amounts and
the water quality of the irrigation water in the channel, irrigated loads were
128
H.L. Fro ¨hlich et al.
within the lowland areas of the catchment is mainly fed by run-off, interflow and
baseflow from irrigated rice fields and fish ponds. From July to September, when
the buffer capacity in the reservoir is exceeded, reservoir spillover also feeds the
river. Due to their limited storage capacity, irrigation reservoirs can cause hazards
as a result of rainfall variability and irrigation management practices during the
monsoon season (see also Sect. 3.3). Particularly during the monsoon season, a
sequence of heavy rainfall events followed by a typhoon can trigger floods, as water
release cannot be controlled once spillover is activated. This study monitored
discharges released from the reservoir and at the sub-catchment outlet during the
wet season of 2008, and also analyzed total organic C and total N content (i.e.,
particulate and dissolved fraction) during several rainfall events which produced a
wide range of rainfall intensities and magnitudes (Schmitter et al. 2012).
3.4.2 Importance of Reservoir Management and Its Impact
on Carbon and Nitrogen Redistribution
During our study, in the absence of rainfall, the water in the reservoir contained on
average 4.7 Æ 1.2 mg L
À1 organic C and 3.8 Æ 1.6 mg L
À1 total N. The low C/N
ratios observed in the reservoir (Æ2) were most likely the result of in situ production
of C and N during decomposition, as aquatic plants are much lower in C when
compared to terrestrial plants (Beusen et al. 2005) leading also to an accumulation
of mineral N, such as NH 4
+ . However, during rainfall events, measured
concentrations at the outlet of the sub-catchment rose rapidly, and values up to
311.4 mg L
À1 organic C and 52.6 mg L
À1 total N were found. The increase in such
concentrations during rainfall events showed a high variability (Fig. 3.10), and a
combined analysis of the pollutograph and hydrograph profiles at both gauges
indicated the presence of non-point sediment sources along the irrigation channel,
which increased loads in the irrigation water. This was also reflected in changes to
the organic C/N ratios at the outlet, which increased during rainfall events to a value
of 7, as compared to a value of 2 for water from the reservoir.
Fluctuations in organic C and N concentrations in the irrigation channel were
highly dependent on the intensity and magnitude of rainfall, the presence of
antecedent dry weather periods and on the amount of irrigation water released
from the reservoir – including as a result of operational management decisions
made during events (i.e., changes in water regimes). A decrease in water released
from the reservoir during rainfall events did result in higher C and N concentrations
in the irrigation channel when compared to similar events when more water was
released from the reservoir.
A quantification of the several contributory water sources (i.e., reservoir and
overland run-off) and their water quality allowed for the estimation of nutrient loads
irrigated to the paddy fields. On the basis of the estimated irrigation amounts and
the water quality of the irrigation water in the channel, irrigated loads were
128
H.L. Fro ¨hlich et al.