Diffuse contributions dominate over point sources of soluble nutrients in two sub-catchments of the Manawatu River, New Zealand

Figures & data

Figure 1  The Manawatu River catchment showing the water quality and flow recording sites of two study catchments: the upper Manawatu and the Mangatainoka.

Figure 2  Concentrations of soluble nutrients at the Manawatu at Hopelands site in decile flow bins as defined by exceedance percentiles (%ile) for A, soluble inorganic nitrogen (SIN); and B, dissolved reactive phosphorus (DRP). Note: the 1st flow exceedance percentile (1st%ile) is shown as a reference to indicate the range of flows, the maximum recorded flow at Hopelands is approximately 1700 m³/s.

Figure 3  Comparison of soluble nutrient concentrations in the Mangatera Stream upstream (open squares) and downstream (closed triangles) of the Dannevirke Sewage Treatment Plant (STP) discharge for A, soluble inorganic nitrogen (SIN); and B, dissolved reactive phosphorus (DRP). Flow exceedance percentiles (%ile) indicate the range of flows in the Mangatera Stream. The regulatory targets and flows at which these apply are shown for each nutrient by the black solid line.

Table 1  Comparison of annual target loads from individual years with mean target load limits calculated from all years of record for the Manawatu at Hopelands (n=15) and Mangatainoka at SH2 (n=12) monitoring sites between 1989 and 2005.

	Manawatu at Hopelands			Mangatainoka at SH2
	Annual target load (all flows)			Annual target load (all flows)
Water year	SIN	DRP	Deviation from mean target load	SIN	DRP	Deviation from mean target load
1989	325	7.3	+9%	–	–	–
1990	390	8.8	−9%	–	–	–
1991	388	8.8	−8%	–	–	–
1992	–	–	–	–	–	–
1993	198	4.5	+45%	185	4.2	+31%
1994	406	9.1	−13%	296	6.7	−10%
1995	425	9.6	−19%	295	6.7	−10%
1996	389	8.8	−9%	311	7.0	−16%
1997	276	6.2	+23%	224	5.1	+16%
1998	283	6.4	+21%	294	6.6	−10%
1999	264	5.9	+26%	209	4.7	+22%
2000	307	6.9	+14%	265	6.0	+1%
2001	396	8.9	−10%	225	5.1	+16%
2002	317	7.1	+11%	233	5.2	+13%
2003	553	12.5	−54%	374	8.4	−40%
2004	453	10.2	−27%	301	6.8	−12%
Mean	358	8.1		268	6.0
Median	388	8.8		280	6.3
Max	553	12.5	+45%	374	8.4	+31%
Min	198	4.5	−54%	185	4.2	−40%
SD	89	2.0		54	1.2
Target load limits were determined from concentration-based nutrient targets in the Proposed One Plan (0.444 g soluble inorganic nitrogen (SIN)/m^3 and 0.010 g dissolved reactive phosphorus (DRP)/m^3) and are expressed in tonnes per year (t/year). Water years were 1 July to 30 June.

Table 2  Comparison of mean annual target loads from all years of record separated by flow decile bin for the Manawatu at Hopelands (n=15) and Mangatainoka at SH2 (n=12) monitoring sites.

	Manawatu at Hopelands				Mangatainoka at SH2
Flow decile bin	Mean target load (all years SIN)	Mean target load (all years DRP)	Total target load in this flow decile bin (%)	Mean target load in this flow decile bin or less (%)	Mean target load (all years SIN)	Mean target load (all years DRP)	Total target load in this flow decile bin (%)	Mean target load in this flow decile bin or less (%)
0–10th	147	3.3	41	100	123	2.8	46	100
10th–20th	56	1.3	16	59	48	1.1	18	54
20th–30^th	39	0.9	11	43	31	0.7	12	36
30th–40^th	30	0.7	8	32	22	0.5	8	24
40th–50^th	24	0.5	7	24	14	0.3	5	16
50th–60^th	19	0.4	5	17	11	0.2	4	11
60th–70^th	16	0.4	4	12	8	0.2	3	7
70th–80^th	12	0.3	3	8	5	0.1	2	4
80th–90^th	9	0.2	3	4	3	0.1	1	2
90th–100th	6	0.1	2	2	2	<0.1	1	1
All flows	358.0	8.1	100		267.6	6.0	100
Flows less than 20th percentile	155.6	3.51	43.5		96.7	2.2	36
SD	9.5	0.21			7.4	0.17
Target loads were determined from concentration-based nutrient targets in the Proposed One Plan (0.444 g soluble inorganic nitrogen (SIN) /m^3 and 0.010 g dissolved reactive phosphorus (DRP) /m^3) and are expressed in tonnes per year (t/year). These standards apply at all flows less than flood flows (<20th flow exceedance percentile for the site).

Figure 4  Comparison of regulatory target loads with measured loads in tonnes per year (t/year) at all flows and flows less than the 20th flow exceedance percentile for two study sites in the Manawatu River catchment for A, soluble inorganic nitrogen (SIN); and B, dissolved reactive phosphorus (DRP). Error bars = + 1SD.

Table 3  Comparison of the percentage change in measured and target soluble inorganic nitrogen (SIN) and dissolved reactive phosphorus (DRP) loads in tonnes per year (t/year) at all flows and with flood flows excluded for the Manawatu at Hopelands and Mangatainoka at SH2 state of the environment monitoring sites in the upper Manawatu River catchment.

	SIN			DRP
	Measured load (t/year)	Target load (t/year)	Measured load greater than target load (%)	Measured load (t/year)	Target load (t/year)	Measured load greater than target load (%)
Manawatu at Hopelands
All flows load	745	358	108%	20.6	8.1	154%
Load excluding flood flows	358	156	129%	8.6	3.5	146%
All flows load greater than load excluding flood flows (%)	108%	129%		140%	131%
Mangatainoka at SH2
All flows load	603	268	125%	9.3	6.0	55%
Load excluding flood flows	281	97	190%	2.9	2.2	32%
All flows load greater than load excluding flood flows (%)	115%	176%		221%	173%
Flood flows = all flows less than the 20th flow exceedance percentile.

Figure 5  Summary of relative contributions of soluble nutrients from point and diffuse sources for the Manawatu at Hopelands and Mangatainoka at SH2 study sites, in comparison with target and measured loads for A, soluble inorganic nitrogen (SIN); and B, dissolved reactive phosphorus (DRP). All loads are expressed in tonnes per year (t/year). Error bars = + 1SD.

Table 4  Comparison of relative loads of soluble inorganic nitrogen from point and diffuse sources at different flows calculated in tonnes per year (t/year) for the Manawatu at Hopelands state of environment monitoring site in the upper Manawatu River catchment.

Flow decile bin	Mean target load (t/year)	Mean measured load (t/year)	Mean point source load (t/year)	Mean non-point source load (t/year)	Point source contribution to measured load (%)	Non-point source contribution to measured load (%)	Point source contribution to target (%)	Non-point source contribution to target (%)
0–10th	146.7	267.4	2	265.4	1	99	1	181
10th–20th	55.7	119.2	2	117.2	2	98	4	211
20th–30th	39.3	104.3	2	102.3	2	98	5	260
30th–40th	30.1	76.5	3.5	73	5	95	12	243
40th–50th	24	65.8	1.3	64.5	2	98	5	269
50th–60th	19.4	45.9	1.9	44	4	96	10	226
60th–70th	15.5	30	2.2	27.7	7	93	14	179
70th–80th	12.3	18.9	0.4	18.5	2	98	3	150
80th–90th	9.2	12.4	1.3	11.1	10	90	14	121
90th–100th	5.7	4.7	0.5	4.2	10	90	9	74
All flows	358	745.1	17.1	728	2	98	5	203
Flows >20th percentile	202.4	386.6	4	382.6	1	99	2	189
Flows <20th percentile	155.6	358.5	13.1	345.3	4	96	8	222
Flows <50th percentile	62.2	111.9	6.3	105.6	6	94	10	170
Flows <80th percentile	14.9	17.1	1.8	15.3	10	90	12	103
Percentage of total load
All flows	100%	100%	100%	100%
Flows >20th percentile	57%	52%	23%	53%
Flows <20th percentile	44%	48%	77%	47%
Flows <50th percentile	17%	15%	37%	15%
Flows <80th percentile	4%	2%	10%	2%

Table 5  Comparison of relative loads of dissolved reactive phosphorus from point and diffuse sources at different flows calculated in tonnes per year (t/year) for the Manawatu at Hopelands state of environment monitoring site in the upper Manawatu River catchment.

Flow decile bin	Mean target load (t/year)	Mean measured load (t/year)	Mean point source load (t/year)	Mean non-point source load (t/year)	Point source contribution to measured load (%)	Non-point source contribution to measured load (%)	Point source contribution to target (%)	Non-point source contribution to target (%)
0–10th	3.30	7.80	0.28	7.52	4	96	9	228
10th–20th	1.25	4.20	0.28	3.92	7	93	22	313
20th–30th	0.89	2.70	0.28	2.42	10	90	32	273
30th–40th	0.68	1.66	0.42	1.24	26	74	62	182
40th–50th	0.54	1.34	0.22	1.12	16	84	40	207
50th–60th	0.44	0.96	0.24	0.72	25	75	54	165
60th–70th	0.35	0.80	0.27	0.53	33	67	76	152
70th–80th	0.28	0.47	0.18	0.29	38	62	65	105
80th–90th	0.21	0.34	0.18	0.17	52	48	86	80
90th–100th	0.13	0.34	0.22	0.12	64	36	170	96
All flows	8.06	20.62	2.56	18.06	12	88	32	224
Flows >20th percentile	4.56	12.01	0.56	11.44	5	95	12	251
Flows <20th percentile	3.50	8.61	2.00	6.61	23	77	57	189
Flows <50th percentile	1.40	2.92	1.08	1.84	37	63	77	131
Flows <80th percentile	0.34	0.69	0.40	0.29	58	42	118	86
Percentage of total load
All flows	100.0%	100.0%	100.0%	100.0%
Flows >20th percentile	56.5%	58.2%	21.9%	63.4%
Flows <20th percentile	43.4%	41.8%	78.1%	36.6%
Flows <50th percentile	17.4%	14.1%	42.1%	10.2%
Flows <80th percentile	4.2%	3.3%	15.5%	1.6%