Scales in Hvdrotogv and Water Management / Echelles en hydrologie et gestion de l'eau (IAHS Publ. 287. 2004)

61

Multiscale approaches to watershed management: land-use impacts on nutrient and sediment dynamics

HANS SCHREIER & SANDRA BROWN Institute for Resources and Environment, University of British Columbia, Vancouver, British Columbia V6T IZ3, Canada

s tar ta j in terchan ge. u be. ca

Abstract Excess nutrients and soil erosion are the two most important non-point sources (NPS) of pollution originating from agriculture. A nutrient mass-balance budget model linked to a GIS was used to quantify excess nutrient applications at three spatial scales: farmers' fields, small watersheds and river basins. While farm based budgets were useful for individual farm manage­ment, the results could not easily be scaled up to the watershed or regional basis due to the very large spatial uncertainties. Using farm census data for all agricultural land in the watershed proved to be more effective, because surplus nutrient applications could be related to stream water quality. Nutrient budgets at the macroscale were not related to water quality but were found to be useful for identifying areas of greatest concern for NPS. Sediment budgets were determined over four different spatial scales and relationships were found to be nonlinear with a reduction of about 30% in annual sediment load between plot scale observations and those measured at the watershed scale. Site varia­bility, land-use activities and vegetation cover were found to be the key factors in determining annual rates of suspended sediment movement. Both of these case studies showed that in order to address NPS pollution a multiple scale approach is more effective than scaling up from small plots or fields to the watershed scale.

K e y w o r d s agr icu l tura l nu t r ien t s ; c u m u l a t i v e effects ; n i t rogen ; n o n p o i n t s o u r c e po l lu t ion ;

nu t r i en t b u d g e t s ; sca l ing ; s e d i m e n t b u d g e t s ; s e d i m e n t y ie ld ; w a t e r qua l i ty ; w a t e r s h e d s

Approches multi-échelles de la gestion des bassins: impacts de l'utilisation du sol sur la dynamique des engrais et des sédiments Résumé Les excès d'engrais et l'érosion des sols, sont les deux plus importantes sources pollution étendue (NPS: nonpoint sources) émanant de l'agriculture. Une modélisation des densités d'engrais appliquées au GIS (Geographic Information System) a été utilisé pour quantifier les excès de l'utilisation d'engrais sur trois échelles: parcelles agricoles, petits et grands bassins hydrographiques. Alors que les équilibres d'engrais (surplus/manque) basés par ferme étaient utiles pour la gestion individuelle des fermes, les résultats ne pouvaient pas être aggréger à l'échelle des bassins ou à une échelle plus grande à cause des importantes incertitudes spatiales. Le recensement des données propres aux terres se trouvant dans le même bassin, s'est avéré plus efficace, car le surplus d'utilisation d'engrais pouvait alors être lié à la qualité de l'eau des ruisseaux. Les équilibres d'engrais à une macro échelle n'ont pas été liés à la qualité de l'eau mais se sont avérés utiles pour identifier les secteurs ou la plus grande menace de pollution étendue. Les équilibres de sédiments ont été déterminés sur quatre échelles différentes, et les relations se sont avérées non linéaires avec une réduction de la charge annuelle de sédiments d'environ 30% entre les observations à l'échelle des

62 Hans Schreier & Sandra Brown

parcelles et celles à l'échelle des bassins. La diversité des terrains, des cultures et de la végétation se sont révélés être le facteur clé dans la détermination des taux annuels du déplacement des sédiments en suspension. Chacun de ces deux cas d'études a montré qu'afin de relater une pollution étendue, une approche multi-échelle s'avère plus efficace que l'aggrégation l'échelle des bassins des observations effectués à l'échelle des parcelles ou des champs. M o t s clefs po l lu t ion é t e n d u e ; po l lu t ion diffusé; équ i l ib res des s é d i m e n t s ; équ i l ib res d ' e n g r a i s ;

b a s s i n - d ' e a u ; d é p l a c e m e n t d e s s é d i m e n t s ; éche l les

INTRODUCTION

Until recently, engineering solutions have dominated watershed management. This included diking, canalization, piping, retaining stormwater in reservoirs, and speeding up the flow path through improved drainage to remove water and pollutants. These traditional approaches are no longer adequate in view of the increases in climatic variability (Haeberli et ah, 1999) and the rapid changes in land use that are taking place in many watersheds. Innovations are needed to improve the management of water in a watershed context, and the focus has now shifted towards a multibarrier approach, which includes assessments of land use, reducing inputs into the watershed, best management practices, and reduction and prevention of activities that affect streamflow patterns and ecosystem health. Some of the changes that are under way represent a complete reversal of what was done traditionally. Table 1 highlights some of the reversals that are taking place; these include recreating rather than draining wetlands, minimizing rather than maximizing imperviousness, increasing rather than minimizing buffer zones, returning streams to their natural channels rather than constructed channels, and infiltrating as much water as possible into the soil rather than putting it into pipes.

Table 1 Innovation in watershed management practices as opposed to traditional approaches.

Traditional approaches Innovative approaches

Creating impervious surfaces Minimizing imperviousness Minimizing vegetated buffer zones Maximizing vegetated buffer zones Draining wetlands Creating wetlands Piping stormwater into streams and channels Detaining stormwater in ponds, and encouraging Piping stormwater into streams and channels

infiltration into soil with infiltration galleries Channelizing streams Turning streams back into natural channels Focus on point sources of pollution Focus on nonpoint source pollution End of pipe treatment Source control Focus on single pollutants Focus on cumulative effects Expanding water supplies Controlling demand Creating dams Removing dams

Increasing and maintaining infiltration rates in soils has many advantages. Not only is more water stored in the soil, but soil also acts as a physical, chemical and biological filter. In the past we have reduced infiltration rates through the construction of many impervious surfaces (parking lots, driveways, etc.) and we have compacted

Multiscale approaches to watershed management 63

the soil as part of most land-use management practices (Kay, 1990). As a result, streams have become flashier, and pollutants applied to the land are reaching the water courses more rapidly. This is compounded by the intensive piping systems we have built to deal with stormwater, the drainage of agricultural land, the loss of protective buffer zones, and the removal of wetlands that normally act as a natural mitigation and filiation system. Many of the rehabilitation efforts now focus on these issues, and the public in many countries no longer accepts the construction of new large dams. In fact, in a number of places dams are being removed in order to allow fish passage and to improve other environmental services provided by free flowing streams (Gleick, 2000; Postel & Richter, 2003).

We have been reasonably successful in addressing point sources of pollution from industrial and municipal sources. This is relatively easy because we usually know where the point sources are, we know the type of pollutants that are released, we can monitor and model pollution dispersal downsfream, and this provides a scientific basis for legal action and enforcement. However, nonpoint sources (NPS) of pollution are much more difficult to quantify, and few of the traditional methods are satisfactory in quantifying the contaminant source and load. To assess the distribution and dispersal of contaminants from NPS is challenging, and providing the scientific basis for successful legal action to take place is even more difficult. The problem is that individual diffuse pollution sources have little impact by themselves and only become of importance when assessing the cumulative effects downstream. Effects are usually not observed until a critical threshold is reached. This is particularly apparent in agricultural watersheds where excess nutrients leach through the soil and lead to widespread eutrophication. In addition, changes in soil surface conditions results in soil erosion, which impacts the sediment, transport dynamics and the aquatic habitat downstream.

The aim of this paper is to examine the scaling problems associated with eval­uating the impact of nutrient and soil losses from agriculture on water quality and watershed management. Nutrient and soil losses are now considered the two most important types of NPS pollution from agriculture. Case study material from two watersheds in Canada and Nepal are used as examples to show that a multiscale approach rather than a scaling up approach is required in order to address the problem.

The challenges we face in quantifying soil erosion and nutrient leaching are many fold, but the most difficult task is to quantify the dynamics of the processes, and to determine how they change over different landscape/watershed scales. The source of the NPS problems can be assessed in a spatial context using Geographic Information Systems (GIS) (Faran & de Boer, 2003). Overlaying different digital aerial imagery allows us to quantify land cover changes over time. However, determination of the changes in input of nutrients and changes in water infiltration rates that are caused by different management practices is more difficult and can only be done by conducting farm surveys, field assessment and/or modelling.

Erosion is even more difficult to quantify because it is only partially visible from aerial images. It cannot easily be quantified spatially due to the episodic nature of erosion processes, the very large site variability, and most of the analysis is retrospective (budgets, sediment surveys and sediment balances after major events). Proactive quantification is usually only possible through modelling.

64 Hans Schreier & Sandra Brown

It is the aim of this paper to discuss some of the problems we face with NPS pollution issues when we try to scale from field size assessments to the watershed scale. Two watersheds are used to illustrate the scaling challenges. Watersheds in the Lower Fraser Valley (LFV) in British Columbia (near Vancouver), Canada, and the Jhikhu Khola watershed in Nepal (near Kathmandu) are used to illustrate the scaling problem associated with surplus nutrient analysis that leads to eutrophication. The scaling problem associated with soil erosion and sediment transport over different special scales is addressed using the same Nepalese watershed.

METHODS AND APPROACHES

Over the past 10 years both watersheds have experienced massive agricultural intens­ification and the problem of eutrophication is now visible in both. Nutrient budgeting models linked to GIS were used to determine where in each watershed, surplus application of nitrogen (N) and phosphorus (P) occur, and water quality monitoring was conducted to independently verify the impacts. In the Lower Fraser Valley watershed, the Agricultural Census Data (Statistics Canada 1991-2001), which is collected every five years and includes detailed data for all farms, was used to determine nutrient budgets for different spatial units recoded by the census within a watershed and within a river basin. The nutrient budgets were determined using a spreadsheet-based model developed by Brisben (1995). It includes a detailed budgeting calculation that determines annual nutrient balance sheets for N and P for all crops grown in the area, and considers nutrient input from all livestock, fertilizers and atmospheric sources for all farms in the region; see Brisben (1995) and Schreier et al. (2003) for model details. The individual farm data is not released by the government but is available in various spatial agglomerations of increasing size, ranging from enumeration sub-areas to enumeration districts. This allows us to determine budgets at different scales. Budgets for N and P were determined first for the enumeration sub-areas within the Sumas River watershed (SRW) which covers 34 255 ha and is a tributary watershed of the Lower Fraser Valley watershed (LFV). The second set of budgets were determined for the entire LFV, watershed (713 100 ha) and included 20 enumeration districts. Data for 328 farms were used in SRW and 5262 farms in the LFV, and the budget calculations were made using the 1991, 1996 and 2001 census data. The N and P budgets were prepared for each of the different spatial units and inputs included manure (from 10 different types and classes of livestock), fertilizer (from farm data) and atmospheric inputs (from field measurements). Uptake of nutrients was calculated for 15 different crop types, and data from the literature was used for losses to the atmosphere by denitrification and soil N-mineralization.

In the case of the Nepalese watershed, no census data is collected and the budget evaluations were made based on farm surveys carried out in 1995 (70 farmers fields) and in 2000 (65 farmers fields). Data for field size, crops grown, yields obtained and nutrient inputs were collected through interviews, and crop and soil samples were analysed for nutrient content. Farm field budget calculations were made for N and P using the method described by Brown (1997), Brown et al. (1999, 2000) and von Westarp (2002). Emphasis was placed on the main staple crops, which included rice,

Multiscale approaches to watershed management 65

maize, potatoes and tomatoes grown in the individual fields. Attempts were then made to scale up to the watershed scale using statistical and GIS techniques, but this proved to be highly unreliable due to the large spatial variability.

To predict erosion and sedimentation over different spatial scales is equally challenging. Erosion data was collected from five erosion plots (100 m~ size) in the Jhikhu Khola watershed in Nepal on an event basis between 1993 and 2000, and annual erosion rates were calculated for each plot. Three of the plots were located on sloping land under rainfed agriculture, and two plots were on sloping land that is degraded (formerly grazing land) (PARDYP, 2002). Automated tipping bucket rainfall gauges were located at each plot, and the runoff and associated sediment from the plot were collected for all major storm events. Annual rates of water input, runoff and sediment losses were determined. Four hydrometric stations were established in the watershed in such a way that a nested evaluation of the sediment dynamics could be made over different scales (Fig. 1). The sediment losses were measured below the erosion plot (100 m 2) and at hydrometric stations in the mini watershed (72 ha), the sub-watershed (532 ha) and the watershed (11 100 ha). All of these systems are interconnected. A second comparison was made between two gauged sub-watersheds (Dhap and Andheri Khola) to determine the effect of different land use on sediment yield. Annual sediment budgets were determined for the erosion plots, the mini-, the sub-, and the full watershed and sediment rating curves were used to compare the two sub-watersheds. In all cases sediment rating curves and sediment budget calculations were made using the method described by Carver (1996). Sediment yield comparisons were made between the different stations draining different spatial units that range over six orders of magnitude in size. A comparison was also made between sediment contribution from cultivated land and degraded land. These scaling effects were then compared with sediment yield data from other catchment studies in the Himalayan region.

Fig. 1 Nested sampling design for multiscale assessment of sediment transport in the Jhikhu Khola watershed.

66 Hans Schreier & Sandra Brown

RESULTS AND DISCUSSION

Nutrient budgets and scaling

Over the past five years both the JKW and the LFV watersheds have undergone very rapid agricultural intensification. However, as shown in Table 2, the intensification has been very different between the two watersheds. In Nepal the intensification has been in the form of increased crop rotations, which changed from an average of less than 2 rotations per year in 1994 to 2.8 per year in 2000. To sustain these higher rates of production, fertilizer inputs had to be increased dramatically, while the animal numbers did not increase significantly over the same time period. In contrast, in the LFV watershed livestock intensification (particularly chicken production) has been significant. It should be noted that the land areas under cultivation have not changed significantly in both watersheds due to the fact that agricultural land in the LFV is in a land reserve and in the JKW almost all available land is already under cultivation.

Table 2 Agricultural intensification in the three watershed used in the case studies.

Indicators of Sumas watershed Lower Fraser Valley Jhikhu Khola intensification (SRW) Canada, (LFV) Canada, (JKW) Nepal,

343 km 2 7131 km 2 111 km 2

1996-2001 1996-2001 1990-2000 Cattle numbers +18% + 17% - 5 % Pig numbers +68% - 1 5 % Chicken numbers + 104% +87% Average annual crop 2.0-2.8 rotations rotations per year

Nutrient budgets determined for the SRW watershed showed that the overall annual surplus for the watershed was 139 kg N ha"1 of arable land per year in 1996 and 140 kg N ha"' of arable land per year in 2001. The P surplus was 72 kg P ha"' year"1 in 1996 and 70 kg P ha"' year"' in 2001. Despite the increases in stocking density the surplus values have remained more or less the same because, with the awareness of the eutrophication problem in the early 1990s, best management practices have been intro­duced and a limited amount of chicken manure is now exported from the watershed to nutrient deficient areas in the valley. However, the overall surplus is still significantly above what are generally considered tolerant surplus rates: 50-75 hg N ha"1 year"1 and 25-30 kg P ha"' year"'. The watershed was then divided into eight sub-enumeration areas that closely matched the sub-watershed boundaries. The water quality in the stream channel draining each sub-enumeration was analysed eight times over the annual cycle to determine the status and the seasonal variability of water quality. The annual surplus or deficit application rates (N rates above those required by crops, and considering denitrification and atmospheric losses) were then detennined for all eight sub-enumeration areas. The results shown in Fig. 2 indicate that an N deficit of 21 kg N ha"1 year"' was obtained for the headwater area of the watershed and a surplus of 314 kg N ha"' year"' was determined for the area with the highest agricultural intensity (highest animal density and highest use of fertilizer per ha). A correlation analysis

Multiscale approaches to watershed management 67

Fig. 2 Surplus nitrogen applications in the Sumas River watershed (SRW).

1.2

0.2 ' Ammonia — - r = 0.76

• DO - r = -0.84

12

10 ~ U)

£

Sx x O

+ 4

100 200 300 400

Surplus N (kg / cropped ha)

Fig. 3 Significant positive relationship between surplus N applications and ammonia, and significant negative relationship between surplus N applications and dissolved oxygen concentrations, in streamwater in the SRW during the winter period.

(Spearman Rank Correlation) between the N surplus applied to the land and the water quality data was then made. The results showed that there is a positive correlation between surplus N in the different contributing areas and ammonia concentration in streamwater (R = 0.76) and significant negative correlation between surplus N and dissolved oxygen (R = -0.84) during the winter months between November and February (Fig. 3).

68 Hans Schreier & Sandra Brown

Applied-N surplus / deficit 2001 f )

Fig. 4 Differences in surplus nitrogen applications in the LFV at the enumeration district scale level.

The same procedure was used to determine the surplus N and P for the LFV watershed, which is 20 times larger than the SRW. This larger watershed was divided into 20 enumeration districts and the results showed that the overall surplus N in the Lower Fraser Valley watershed was 73 kg N ha"' year"1 in 1991 and 55 kg N ha"1 year"1

in 2001, and the P surplus was 42 kg P ha"1 year"1 in 1996 and 31 kg P ha"1 year"1 in 2001. As shown in Fig. 4, the detailed analysis of the 20 enumeration districts showed values ranging from a deficit of 44 kg N ha"1 year"1 in one district to a surplus of 174 kg N ha"1 year"1 in the most intensively used district. Similarly, the differences in P ranged from a deficit of 66 kg P ha"1 year"1 to a surplus of 92 kg P ha"1 year"1.

No significant correlations could be found between surplus applications and water quality data at that scale because: (a) only 11% of the LFV is in agricultural land as opposed to 54% in the SRW, (b) the range of surplus values were significantly lower in the LFV than in the SRW as related to the changes in resolution, (c) the proportion of water originating from forest that introduces limited amounts of nutrients is likely diluting the nutrients in the LFW, and (d) the nutrient input patterns changed in an uneven manner in the downstream direction, which provides opportunities for nutrient reduction through the many functions provided by the ecosystem.

Similar changes in nutrient dynamics can be shown in the JKW in Nepal. Changes in nutrient inputs were determined from farm surveys and field analysis of 70 fields in

Multiscale approaches to watershed management 69

Table 3 Changes in fertilizer applications based on farm interviews in the Jhikhu Khola Watershed in Nepal.

Irrigated 1995 2000

Rainfed 1995 2000

Fertilizer Type Urea (kg ha"1 year"') 230 460 550 650 DAP (kg ha"' year"1) 100 1190 450 520 Complex (kg ha"1 year' 1) 600 10 20 95 Nutrient input Total N input (kgN ha"' year"1) 455 610 605 775 Total P input (kg P ha"' year"') 105 245 120 155

DAP = diammonium phosphate

1995 and 65 fields in 2000 (von Westarp, 2002). The results provided in Table 3 show that the average nutrient inputs (manure, compost plus fertilizers) have increased substantially over the past five years but there are great differences between irrigated and rainfed agricultural fields, and there has also been an increase and a shift in ferti­lizer amount and type. The largest amount of fertilizer and the largest increase in ferti­lizer applications has occurred in the irrigated fields because it is in these fields that the greatest number of annual crop rotations are possible and where the greatest increases in yields can be obtained. The irrigated fields are usually located in the lower part of the watershed and the rainfed fields and cultivated terraces are in the upland portions of the watershed.

Nutrient budgets were determined for each field for 1995 and 2000 (von Westarp, 2002). Only intensively used fields with the same dominant crop rotation were chosen for this comparison. Budgets were calculated for rice-potato-maize rotations in irrig­ated fields and maize-potato-tomato rotations in rainfed fields. Determining individual farm field budgets is useful to individual farmers because it provides them with an understanding about the effectiveness of the nutrient regime in use. Nutrient deficit values for each field impact crop yields, and surplus applications result in environ­mental pollution and increased costs.

The results provided in Fig. 5 showed that the median field receives insufficient N in irrigated sites but significant N surpluses in rainfed sites. In contrast median annual P applications are in surplus in both irrigated and rainfed sites. With the exception of nitrogen in irrigated sites, all other budget surpluses have increased significantly between 1995 and 2000, and the increase in P surplus has been most dramatic in irrigated sites where the median surplus increased from 25 to 110 kg P ha"1 year"1. The main reason for this is that after democracy was established in Nepal in 1993, high concentrate phosphate fertilizers became more readily available through foreign aid projects. This is clearly evident from Table 3 which shows that between 1995 and 2000 most fanners that irrigated their crops switched from Complex fertilizer to diammonium phosphate (DAP) fertilizer which is much higher in P content and was applied in greater amounts.

What is apparent from Fig. 5 is the great variability that exists between the nuttient budgets of individual fields. This makes scaling up to the watershed level extremely difficult, because it is estimated that there are between 50 000 and 60 000 individual fields on hundreds of terraces in the watershed. In addition, the average farm holding is

70 Hans Schreier & Sandra Brown

(a) 4 0 0

3 0 0

"or JZ

2 0 0

O) J £ 1 0 0

•z (/) 0

a 3 - 1 0 0

CO

- 2 0 0

- 3 0 0

(b) 4 0 0

3 0 0 '(0 £

2 0 0

a. 0)

1 0 0 a.

cn 0

- 1 0 0

M a x i m u m

M i n i m u m

M e d i a n

1 9 9 5 2 0 0 0

I r r i g a t e d

1 9 9 5 2 0 0 0

N o n - i r r i g a t e d

M a x i m u m

M i n i m u m

M e d i a n

1 9 9 5 2 0 0 0

I r r i g a t e d

1 9 9 5 2 0 0 0

N o n - i r r i g a t e d

Fig. 5 Differences and trends in N and P nutrient budgets for intensively used fields in the Jhilchu Khola watershed in Nepal, 1995-2000.

less than 1 ha, and a typical farmer owns at least 4-5 small plots of land that are widely dispersed within the watershed. The data collected from the 65 fields thus represent approximately 0.1% of the total population of fields in the watershed, and given the large variations in budget values between farms we have little confidence in scaling up to the watershed level and determining watershed scale budgets. One possible option would be to conduct a land-use classification from high-resolution satellite imagery and then apply the median budget values to the different types of crops. However, this is complicated by the fact that the agricultural intensification has resulted in triple annual crop rotations, which means three land-use classifications would have to be made over a single year, and not all crops can easily be differentiated from each other through remote sensing data. Even with such an effort it is unlikely that the accuracy of such calculations can be significantly improved.

Over the study period, eutrophication in the Jhikhu Khola River was observed periodically, particularly in the lowland section of the watershed where irrigation is most widespread. However, water quality data could not be linked to the areas where surplus nutrient budgets occur because of the widely dispersed locations of the fields. What these two comparative studies show is that nutrient budgets at the farm scale are

Multiscale approaches to watershed management 71

useful for improving the management of nutrients at the field and farm level, and this can only be scaled up effectively if summarized farm data from a complete census is available for the watershed. The additional problem with using the census is that enumeration areas and enumeration districts rarely follow watershed boundaries. How­ever, the watershed evaluation based on the census data does provide important information on the location of where the largest surplus areas are. If we hope to mitigate nonpoint sources nutrient problems in agriculture, we should probably work at both the farm and the watershed scales.

Sediment budgets and scaling

Erosion and sediment transport processes are extremely dynamic and pose a particularly difficult problem for modelling over different scales because of the episodic nature of events and the fact that different processes occur at different scales. Most sediment budgets are derived from models and few budgets are properly calibrated. Soil losses at the plot scale were related to sediment losses at the sub-, and watershed scales in the JKW in Nepal. Three erosion plots (100 m" in size) were established in 1993 on sloping agricultural land (north and south facing and in the upland section of the watershed), and two plots were established in 1997 on degraded sites (common grazing areas, north and south facing, in the lower section of watershed). The annual erosion rates are highly variable between years and between sites. In fact the site variation was in some cases greater than the seasonal variability. The episodic nature of erosion events is evident in that three major storms per year produce about 70-80% of the annual sediment yield. Of particular interest is that the major erosion events primarily occur during the pre-monsoon season (end of dry season) when the land is usually barren and farmers are waiting for the first rains in order to cultivate the fields. The average annual erosion rates (over eight years) from the agricultural fields were 14.5 t ha"1 year"1 (range 2-37 t ha"1 year"') and the rate from degraded sites was 20.5 t ha"' year"' (range 6-39 t ha"1 year"'). The plots from the degraded sites contribute approximately 1/3 more sediment than the agricultural sites and as a result it is critical to differentiate these two types of sediment sources within the watershed.

The erosion rates from the plots were then compared to sediment budgets determined for suspended sediments by Carver (1996) for the mini-, sub-, and water­shed scale. The results of this nested scale approach are presented in Table 4, which shows that there is a significant reduction in suspended sediment delivery as we move from the plot to the micro watershed scale. What is of particular importance is that the scaling process is not linear and that the reduction in suspended sediments between the plot and the watershed is about 1/3 as a result of sediment deposition and retention processes within the channel and flood plain systems, and sediment diversions into the irrigation system. The uncertainties in predicting erosion rates at the plot scale are particularly high because land surfaces conditions continuously change. When soils are covered by vegetation the risk of erosion is significantly lower and in agriculture the period of soil exposure is short under traditional annual single crop rotation. With three crop rotations per year, the soil exposure time increases and when soils are degraded,

72 Hans Schreier & Sandra Brown

Table 4 Differences in sediment budgets over different spatial scales in the Jhikhu Khola watershed.

Scale Size of drainage within JKW (ha) Annual sediment budgets (t ha"1 year"')

Erosion plot 0.01 14.5-20.5 Mini watershed 72 17 Sub-watershed 532 15 Watershed 11 100 11

1 0 0 0

E

0 .1

• A n d h e r i - - - r ! = 0 . 5 9

D h a p — ! = 0 . 3 0

"

1 1

0 . 0 1 0 .1 1 10 1 0 0

D i s c h a r g e ( m 3 / s e c )

Fig. 6 Differences in sediment rating curves between two watersheds of similar size but different extent of land degradation.

the soils are permanently exposed to rainfall events. There is approximately a 30% lower erosion rate on agricultural plots than on degraded plots, and this can also be shown when we compare two similar sub-watersheds (Lower Andheri vs Dhap Khola). These two sub-watersheds are approximately the same size and have the same amount of agricultural land, but the Dhap Khola has 25% of land that is heavily degraded while only 15%o of the area in the Lower Andheri is degraded. Two erosion plots on degraded sites were located in each of the two sub-watersheds, and there was a slightly lower average annual erosion rate in the Andheri plot than in the Dhap plot (26 t ha"' year"1 vs 20 t ha"' year"'). A comparison was made between the pre-monsoon sediment rating curves of the two sub-watersheds (Fig. 6) and the results show that the suspended sediment rates are significantly greater at lower flow rates in the more degraded Dhap Khola than in the Andeheri Khola, but at high flow rates the two rates become similar. This suggests that as the proportion of degraded land in a sub-watershed increases, the more sediments are removed at low flow rates. Although no sediment budget was produced for the Dhap Khola because of the lack of continuous flow data, the sediment-rating curve clearly indicates that the Dhap Khola has a higher annual suspended rate than the Andheri Khola. One of the key soil indicators that impacts sediment delivery is hydraulic conductivity and this is one of the most highly variable soil properties. Not only is it highly variable because of differences in inherent soil geological and topographic conditions, but because the rates are constantly changing due to soil management practices and changes in vegetation cover. Erosion plot monitoring, even with the use of sophisticated automation, is time consuming and it is unrealistic to set up a plot network in a watershed that is truly representative of the

Multiscale approaches to watershed management 73

different conditions. As a consequence, estimating erosion rates and sediment budgets over different scales requires a combination of approaches that includes the use of models based on topography, site conditions and land use, and the determination of sediment yields using sediment rating curves. This again requires assessment at different scales.

To show the complexity of sediment yields over different watershed scales, data reported in the literature of suspended sediment yields in the Nepalese Himalayas were compiled in Fig. 7. The results show highly variable rates over different scales and one could even suggest that very different relationships exist in those catchments of <2000 km 2 size compared to those >10 000 Ion2. It could be argued that the bigger the watershed the more the sediment rates are dominated by the erosion and re-suspension process of sediments from stream banks and from flood plain deposits, while in small catchments surface erosion has a more profound impact. However, a lot more sediment data from different size watersheds is needed to lend more credence to this argument.

100000

Watershed size in km

Fig. 7 Relationships between watershed size and suspended sediment delivery at different watershed scales (based on literature data compiled by Galay et al, 2002).

CONCLUSIONS

Nutrient and sediment inputs into streams are rapidly emerging as the most widespread problems facing watershed managers. In the two case studies presented in this paper it was shown that excess nutrients and sediments, which are the two key sources of NPS pollution, are difficult to quantify and a multiscale approach is needed that includes both modelling and in-field measurements at different scales. In the case of N and P budget analyses, it was shown that while farm budgets are useful for individual farmers in order to help them improve the effectiveness of nutrient management and thus prevent excess input into streams, such budgets alone are not sufficient to predict the overall impact on water quality in a watershed. At the intermediate watershed scale, relationships were shown between surplus N and P nutrients applied and ammonia and dissolved oxygen concentration in the water, as long as the watershed is dominated by intensive agriculture. At the river basin scale it was not possible to link the nutrient budgets to water quality, but the linkages of the budgets to the GIS map at

74 Hans Schreier & Sandra Brown

that scale helps in identifying where the greatest surplus applications in the region occurs and where the risk of pollution is the greatest.

Sediment yields are even more difficult to assess because of the constantly changing soil hydrological conditions and vegetation cover due to land management. Plot scale erosion rates were shown to be highly variable between sites, between seasons and between years. The erosion plots (0.01 ha in size) were shown to produce at least 30% more sediments than the sediment yields determined at the watershed scale (11 100 ha size) and the relationships over different spatial scales were not linear. Differences between site conditions and land-use types are the key factors that influence sediment yields at the local scale. Agricultural plots had almost 1/3 less erosion annually than occurred from degraded grazing land that had little vegetation cover. Comparing published sediment yield data for different size watersheds ranging from 0.1 lan 2 to 100 000 km 2 in the Himalayan region (Galey et al., 2002) suggests that there is some corroborative evidence that different processes dominate at different scales. Only a multiple scale approach can discern these effects and significantly more data is needed at different scales to give credence to this concept.

Acknowledgements The sediment budget research in the LFV and SRW in Canada was supported with grants from Environment Canada and the Canadian Water Network. The Nepalese watershed research was supported by grants from the International Development Research Centre (IDRC), Ottawa, Canada, and SDC, Bern, Switzerland.

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