DECLARATION 2- PUBLICATIONS
7.2 Model Calibration and Validation
7.2.2 Calibration of runoff, nutrient and sediment yields
7.2.2.1 Runoff and root zone water balance
The model was run for the period 2006 to 2012 using the measured meteorological data and comparisons were made for the summer period of October 2007 and March 2008.
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Figure 7.2: Hydrologic calibration of ACRU-NPS model for daily runoff for the period October 2007 and March 2008.
The measured and simulated daily runoff from Flume 2, shown in Figure 7.2, indicates that the simulated runoff follows a similar trend as that of measured runoff. From the graphical comparisons (Figures 7.2 and 7.3) it can be inferred that the calibrated parameters for the studied catchment realistically represent the nature and behaviour of the catchment. The marginal differences may have resulted from inaccuracies associated with input data to the model, specifically, subtle differences in channel, soil and subsurface properties (Van Liew and Garbrecht, 2003).
Figure 7.3: Cumulative runoff for observed and simulated runoff (left) and 1:1 comparison between observed and simulated runoff (right).
0
20
40
60
80
100
120 0
3 6 9 12 15
12-Sep-07 13-Oct-07 13-Nov-07 14-Dec-07 14-Jan-08 14-Feb-08 16-Mar-08
Rainfall (mm)
Observed and simulated Runoff (mm)
Time (Days)
Rainfall (mm) Observed Runoff (mm) Simulated Runoff (mm)
0 100 200 300 400 500 600 700
0 15 30 45 60 75 90
Sep-07 Oct-07 Nov-07 Dec-07 Jan-08 Feb-08 Mar-08
Cummulative Rainfall (mm)
Flume 2 Simulation Period (Days)
Cummulative Runoff (mm)
Observed vs Simulated Runoff
∑ Obs. Runoff, mm
∑ Sim. Runoff, mm
∑ Rainfall, mm
y = 0.98x R² = 0.94 0
2 4 6 8 10 12
0 2 4 6 8 10 12
Observed Runoff (mm)
Simulated Runoff (mm)
1:1 Obs. vs Sim. Runoff Linear (1:1 Obs. vs Sim. Runoff)
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The results of the statistical tests outlined in Table 7.6 showed that the measured and the simulated mean runoff was not significantly different at the 95% confidence level during hydrologic calibration of the model as the calculated student’s t-test value was lower than the critical limits (-0.000005< 1.97). The values of R2 (0.94) and NSE (0.87) also indicated agreement between the measured and simulated results. The value of RMSE for daily runoff (0.37mm) showed that the model slightly deviated from the respective measured runoff.
Figure 7.4 shows saturated drainage, baseflow and baseflow storage throughout the ACRU- NPS simulation. During calibration the final value of ABRESP (i.e. the fraction of
“saturated” soil water to be redistributed daily from the topsoil into the subsoil store when the topsoil is above the drained upper limit) was set to 0.60. Final BFRESP value was 0.75 and it represented the fraction of “saturated” soil water to be redistributed daily from the subsoil into the intermediate/groundwater store when the subsoil is above its drained upper limit. For the whole period of simulation there was more cumulated saturated water draining from the A-Horizon to B-Horizon (SUR1) than that draining from the B-Horizon to groundwater (SUR2).
Figure 7.4: Saturated drainage and base flow storage.
At the onset of summer rains in October there was little base flow storage (RUNCO) because saturated drainage was from A-Horizon to B-Horizon only. However, as the rainy season continued (from October towards April) vertical drainage contributions from B-Horizon to the groundwater zone was realised (around December). The increased hydrological connectivity between the two soil horizons (A and B) resulted in more water reaching the
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groundwater store and hence the increase in base flow storage. This later culminated in more base flow being constituted in storm flow as opposed to surface runoff as in the beginning.
Baseflow can be separated from stormflow using isotope techniques.
The results from the root zone water balance (Figure 7.5) indicates that during the simulation period around 14th January 2008 , the soil water contents in horizons A (STO1) and horizon B (STO2) had exceeded the field capacities in both horizons (i.e. FC1 and FC2 = 0.32) thereby resulting in soil surface runoff as confirmed in Figure 7.2. During rainy seasons (October to April), there is more soil water in the A-horizon than in the B-Horizon. Most of the water is transpired from the A- Horizon (ATRAN1) as crop growth proceeds. This in turn results in a more rapid LAI increase (V0GLAI).
Figure 7.5: Root zone water balance for simulation period 2008 -2012.
The onset of plant water stress is determined by a constant of 0.2 (CONST = 0.2). This constant is the fraction of the plant-available water within the soil horizon at which total evaporation is assumed to drop below the maximum evaporation during drying of the soil.
During the early stages of plant growth, sugarcane experiences less water stress because there is an abundance of water. The rapid LAI increase associated with N-uptake from the rapidly growing crop (indicated by a steep slope in V0GLAI) results in more nitrogen stresses
0 1 2 3 4 5 6 7 8
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45
Jan-08 Jul-08 Jan-09 Jul-09 Jan-10 Jul-10 Jan-11 Jul-11 Jan-12
ATRAN, V0GLAI, V0NSTFAC
FC, STO, WP, PO
FC1 FC2 STO1 STO2
WP1 WP2 PO1 PO2
ATRAN1 ATRAN2 V0NSTFAC V0GLAI
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(V0NSTFAC) (Figure 7.5). At later stages of the season, however, rainfall decreases along with the soil water content in the A- Horizon. Since the crop has now developed deeper roots it is forced to transpire more B-Horizon (ATRAN2) water. This eventually results in reduced LAI growth (indicated by flatter slope) and rapid depletion of soil moisture contents in both horizons. The trend is shown in Figure 7.5 as moisture contents STO1 and STO2 approach the wilting points WP1 = 0.11 (A-Horizon) and WP2 = 0.11 (B-Horizon). The soil porosities for A-Horizon (PO1) and B-Horizon (PO2) were both 0.43 which indicates the soil water content at saturation.