Mapping the Environmental and Economic Cost of Soil Loss on the Resiliency of Abia State Hydrological Basin: A study between 1972 and 2015

1972 and 2015 as well as its agro-productivity, socio economic equalities and overall well-being of Abia sate. This research highlights the fact that proper conservation measures needs to be applied to improve agro productivity, water quality standard and the general well-being of Abia state.


Introduction
Soil erosion by water is a major problem in the world (Lant et al., 2015), with economic, social and environmental implications arising from both on-site and off-site effects (Engel et al., 2008).Soil erosion damages capital that supports economic development and improvements in quality of life.Human-induced conversion of natural capital reduces future service flows, unless the capital is restored when degraded (Blignaut et al., 2007).According to Herath (2001), the losses of soil in terms of the irreplaceable inputs are the primary factors determining the productivity and economic costs of erosion.The causes and consequences of soil erosion are far from being settled.Inappropriate government policies and institutions, commodity prices, farm subsidies, taxes and other forms of government intervention have all being implicated in soil erosion.Lipton (1987) argued that high commodity prices would encourage "soil mining" for quick and bigger crops now.LaFrance (1992) concludes that where both cultivation intensity and the level of conservation activity responds to market forces, higher prices lead to more intensive use of soil thereby aggravating soil erosion.Clarke (1992) found that investment on soil conservation measures would increase when product prices are favorable and that economically viable conservation technologies are available.World Bank's world development report (1986) states that the poor performance of agriculture in low-income countries is due to macroeconomic policies such as overvalued exchange rates and agricultural taxes which alter incentives for farmers.Chisholm et al.(1997) examined Sri Lanka's trade liberalization and cautioned that economic losses from soil erosion in Sri Lanka are quite substantial even under low erosion-low economic impact assumptions and that trade reforms alone are inadequate to substantially reduce soil erosion.They contend that policies, which directly target soil erosion, are required to minimize social losses from such erosion.
Understanding the economic costs of soil erosion is vital for farmers, soil conservation experts and policy makers in order to put the potential benefits of soil conservation practices into context (Shiferaw et al., 2005), and to set priorities for land management practices (Igwe & Fukuoka, 2010;Kidane & Alemu ,2015).Soil erosion produces both on-site and off-site damage such as water pollution and sedimentation of waterways (Herath, 1985).Some off-site effects can be beneficial but often damaging effects are apparent.These externalities persist due to market failure caused by the absence of property rights.The costs of soil erosion and the benefits of soil conservation are difficult to determine.
However, erosion and sedimentation are natural processes that contribute to healthy ecosystems, but too much may have severe consequences.Excessive erosion can reduce agricultural productivity, increase flooding and pollutant transport, and threaten bridges, railroads and power infrastructures.Erosion can lead to sediment build-up, which strains water infrastructures, such as reservoirs and flood control systems, and increases water treatment costs.Sedimentation is particularly problematic for reservoirs, which are designed to retain sediment as water is released (Tallis et al., 2013).Regular sediment removal can avoid some of these issues but this involves expensive maintenance costs.The magnitude of sediment transport in a watershed is determined by several factors.Natural variation in soil properties, precipitation patterns, and slope create patterns of erosion and sediment runoff.Vegetation holds soil in place and captures sediment moving overland.However, changes in land management practices can alter the sediment retention capacity of land by removing important vegetation (Tallis et al., 2013).To reduce the damages and costs associated with sedimentation, land, water and reservoir managers require information regarding the extent to which different parts of a landscape contribute to sediment retention, and how land use changes may affect this retention.InVEST model aims to provide these kinds of information.The outputs from these models will allow planners and managers to consider how LULC (Land Use/Land Cover) change in one area in the basin can cause sedimentation problems at other locations.Also, the model provides information for non-point source pollutants and deal with nutrient pollutants (nitrogen and phosphorous) lost and retention.For the development of policy, oriented towards sustainable development of agriculture, quantitative assessment of the on-site and off-site damage due to erosion was performed.

Material and Method
This research is interested in estimating the cost of soil erosion (on/off-site damage) using InVest model adopting GIS (Geographic Information System) technique.The on-site costs were estimated on the basis of soil and nutrients loss.Offsite costs were estimated using sedimentation.Data collected for this study include: (1.) Field survey, this entails taking GPS reading and ground trotting of study locations; (2.) USGS (U.S.  (NRCRI), Umudike between 1972 and 2015 (a span of 43 years).NRCRI is located between 5° 29 1 North to 7° 33 1 East on an elevation high of 122m in Abia state.Also, current and relevant literatures were reviewed for Abia state with regards to the subject matter.The study extent covers Abia state and is located east of Imo state and shares common boundaries with Anambra, Enugu and Ebonyi states to the North West, North and North East respectively (Nwilo et al., 2011).To the East and South East, it is bounded by Cross River and Akwa Ibom states and by Rivers state to the South.It occupies a landmass of 5,833.77square kilometers.Abia state is located on longitude 7° 09 1 to 8° 05 1 East and latitude 4° 48 1 to 6° 03 1 North (Figure1).Abia state comprises of seventeen ( 17  Acquired analogue base maps were scanned and converted to digital/raster images.The scanned maps were georeferenced and projected to UTM 32N and digitized in ArcGIS 10 software.Also, attribute tables and values was created and used to carrying out the necessary GIS analysis/manipulation and operations with a view to produce relevant maps, reports, charts and tables.Bands 4, 5 and 7 of the acquired Landsat MSS for 1972, TM 4 for 1986, ETM+ 7 2003 and OLI 8 for 2015 with a resolution of 30m were selected, mosaic and enhanced.Supervised classification method were adopted to define training sites, extract signatures and classify the remotely sensed imagery using maximum likelihood classification procedure into six classes, namely: Agriculture land, bare ground, primary forest, secondary forest, built-up area, wetland and water body, this was implemented using Idrisi Selva software.ArcGIS10 Arc Hydro extension was used to perform drainage analysis on a terrain model.The output was used to generate Abia state basin which was used as input Sediment Retention and Nutrient Retention modeling.

Water. Environ. Sustainability. 3 (2): 1-21, 2023
In this study, Sediment Retention model was used to calculate the average annual soil loss from each parcel of land, determining how much of that soil may arrive at a particular point of interest, estimating the ability of each parcel to retain sediment, and assessing the cost of removing the accumulated sediment on an annual basis, the model uses the Universal Soil Loss Equation (USLE) (Wischmeier & Smith, 1978) at the pixel scale, which integrates information on LULC and soil, as well as a digital elevation model and rainfall.Universal Soil Loss Equation (USLE) provides the foundation of the biophysical step of the InVEST sediment retention model (version 2.6, a toolset of ArcGIS 10 software): Where, R is the rainfall erosivity, K is the soil erodibility factor, LS is the slope length-gradient factor, C is the crop-management factor and P is the support practice factor.
Rainfall erosivity index (R) (required).R is a GIS raster dataset, with an erosivity index value for each cell.This variable depends on the intensity and duration of rainfall in the area of interest.The following equation is widely used to calculate the R Index:
Appropriate P and C factor were added to the land use/ land cover class and stored as tables in the attribute table as biophysical table.The support practice factor, P, accounts for the effects of contour plowing, strip-cropping or terracing relative to straight-row farming up and down the slope.The cover-management factor, C, accounts for the specified crop and management relative to tilled continuous fallow.C factor for Abia state was obtained from the LULC for 1972LULC for , 1986LULC for , 2003LULC for and 2015 based on existing literature (Roose, 1977).The support practice (P) has a value between 0 and 1 for each land use in Abia state.Ground trotting was carried out to confirm that there were no measurable conservation measures in Abia state.
The slope threshold that the model uses to switch between the following two equations is specified as a model input and depends on the local geomorphology and basin characteristics.For low slopes: Where, flowacc = Accumulated water flow to each cell and cellsize = Pixel size or the grid resolution (10m, 30m, 90m, etc.).For high slopes: We use the following equation, defined by Huang and Lu (1993) for areas with slopes higher than the threshold identified by the user: Where, prct_slope = Pixel's percent slope and flowdir = Flow direction of the pixel.This research estimates the ability of vegetation to keep soil in place on a given pixel by comparing erosion rates on that pixel to what erosion rates would be on that pixel with no vegetation present (bare soil).The bare soil estimate is calculated as follows: Erosion from the pixel with existing vegetation is calculated by the USLE equation: Avoided erosion (sediment retention) on the pixel is then calculated by subtracting USLE from RKLS.The potential soil loss was classified based on the results gotten from USLE (R L S K C P) computation above using table 1. Avoided erosion (sediment retention) on the pixel is then calculated by subtracting USLE from RKLS.Vegetation does not only keep sediment from eroding where it grows.It also traps sediment that has eroded upstream.The USLE equation overlooks this component of sediment dynamics, so we attempt to account for it as follows.All soil that the USLE equation estimated is eroded routed downstream via a flow path (Tallis et al., 2013).However, sediment eroded is trapped by downstream vegetation thereby retaining sediment.The model also determines the total sediment load exported that reaches the stream from each pixel on the landscape (Tallis et al., 2013).The total retained sediment ( ) is equal to the sum of the sediment removed by the pixel itself and the sediment removed through routing filtration (Tallis et al., 2013).The model provides the option to consider two services associated with the retention of sediments on the landscape; improved water quality and avoided sedimentation of reservoirs (Tallis et al., 2013).We assume that each pixel on the landscape gets an equal proportion of this allowance in the following calculation: The InVEST Water Purification Nutrient Retention model (version 2.6, a toolset of ArcGIS 10 software) was used to calculate the amount of nutrient retained on every pixel then sums and averages nutrient export and retention per basin.The pixel-scale calculations allow us to represent the heterogeneity of key driving factors in water yield such as soil type, precipitation, vegetation type, etc (Tallis et al.,2013).The model used Reckhow et al. (1980) export coefficients and annual averages of pollutant fluxes derived from various field studies that measure export from parcels this was done using the following equation: Where, ALV x = Adjusted loading value at pixel x; pol x = Export coefficient at pixel x; and HSS x = Hydrologic sensitivity score at pixel x which is calculated as: Where, λ x = Runoff is index at pixel x, calculated using the following equation, and  ̅  = the mean runoff index in the watershed of interest.
( ) Where, ∑ Y u u = Sum of the water yield of pixels along the flow path above pixel x (it also includes the water yield of pixel x).The model helps us model how much pollutant leaving each pixel and determine how much of that load that is retained by each downstream pixel, as surface runoff moves the pollutant toward the stream (Tallis et al., 2013).
To calculate the amount of service delivered, the model decreases retention by the amount of 'allowed' pollution in the water body of interest, if an allowed amount is given (Tallis et al., 2013).If a threshold is given, the service level is calculated in biophysical terms as follows: The nutrient retained is determined, for each basin based on the avoided treatment costs that retention by natural vegetation and soil provides (Tallis et al., 2013).
We make this calculation as follows:   In forest areas, a minimum of 60% forest cover is necessary to prevent serious soil erosion (Singh & Kaur, 1989;Haigh et al., 1995;Forest Conservation Act, 2002) but in Abia state it ranges from 6 to 36% with a soil loss of 218.70tons/basin to 86,822.68 tons/basin.Figure 10 shows the map of nitrogen retained and figure 11  Figure 12 shows the map of phosphorous exported and Figure 14  Figure 13 shows the map of phosphorous retained and figure 14  than the remaining soil left.Furthermore, it was found that phosphorus is apparently the most deficient plant nutrient eroded in the soil than nitrogen in Abia state hydrological basin.A total estimate of 22.00% to 40.69% of the soils nutrient is retained and 88% to 59.31% are loss for nitrogen and 22.34% to 37.96% for phosphorous is retained while 77.66% to 62.04% is loss between 1972 and 2015.This affects water quality, plant growth and overall agro-productivity.A deficit in soil nutrient between 1972 and 2015 with phosphorous leading the way by 62.04% against 59.31% of nitrogen was observed in Abia state hydrological basin.This accounts for 1% of the sediment discharged as export by the landscape against the 99% retained in Abia state.The exported sediment however small still has an impact on Abia state hydrological basin, based on this; the effects LULC change on nutrient loss was studied.Figure 15 shows the graphical details of the distribution of phosphorous exported in Abia state hydrological basin for 1972, 1986, 2003 and 2015.All classes recorded minimum and mean phosphorus of 0.00 kg/basin for 1972, 1986, 2003 and 2015, with a mean of 0.00 kg/basin for 1972.For 1986 and 2015, agricultural land, wetland, secondary and primary forest recorded mean phosphorus of 0.00 kg/basin expect for wetland in 2015 with a mean of 0.01 kg/basin.In 1972, for agricultural land phosphorus export was observed to be low with a maximum of 0.57 kg/basin.Built up areas was observed to export maximum phosphorus of 0.59 kg/basin and mean of 0.05 kg/basin in 1972.Bare ground was observed to export maximum of 0.52kg/basin and mean of 0.01 kg/basin in 1972.Secondary forest was observed to export a maximum of 0.51 kg/basin of phosphorus in 1972.In primary forest, maximum of 0.58kg/basin was exported in 1972.Water body was observed to export a maximum of 0.57kg/basin of phosphorus in 1972.In 1986, for agricultural land phosphorus export was observed to be low with a maximum of 0.59 kg/basin.For built up areas, maximum of 0.66 kg/basin and mean of 0.16 kg/basin was exported in 1986.Bare ground, maximum of 0.66 kg/basin and mean of 0.07 kg/basin phosphorus was exported in   Primary forest a maximum of 0.62 kg/basin of phosphorus was exported in 2015.Wetland, maximum of 0.58kg/basin of phosphorus was exported in 2015.Water body was observed to export a maximum of 0.29kg/basin of phosphorus in 2015.The result reveals that LULC change in one area of basin can cause nutrient retention problems at other locations.Also, it was discovered that LULC with no filtering capacity have high export capacity, such as build-up areas, bare ground and water body.Thus, these are areas where investments in protecting this environmental service will provide the greatest returns and also where land use changes may have the greatest impacts on service provision.

Effect of Land use change on Nutrient Export in
Figure 14.Phosphorous exported and retained for Abia state in 1972, 1986, 2003 and 2015.

Economic benefits and loss of Nutrient Retention on Abia state Hydrological Basin
Abia state hydrological basin is part of the Nigeria's hydrological basin, which supplies drinking water for Abia residents.Natural landscape characteristics, agronomic and economic conditions result in high nutrient (Phosphorus and Nitrogen) transport to the reservoir in amounts that affects current water quality standards.Environmental cost of water purification was computed and found to have a high economic value due to deforestation and high nutrient runoff in Abia state.Variation in the results has indicated that land use changes may have the greatest impacts on service provision and water purification.Figure 16 show the economic benefit of retaining nitrogen in 1972, 1986, 2003 and 2015 for Abia state hydrological basin.Figure 18 graphically shows the economic value of exporting and retaining nitrogen in 1972, 1986, 2003 and 2015 for Abia state hydrological basin.For Imo river basin in 1972, the economic value of nitrogen exported was found to be $34328.00and $23551.20 for nitrogen retained with $7667290.00as the economic benefit (value) of the basin for retaining nitrogen over 14 years span.For Ikwa Ibo river basin in 1972, the economic value of nitrogen exported was found to be $10378.50and $6172.31for nitrogen retained with $2318090.00as the economic benefit of the basin for retaining nitrogen over 14 years span.For Enyong creek sub basin in 1972, the economic value of nitrogen exported was found to be $16258.30and $10727.30for nitrogen retained with $3631355.06as the economic benefit of the basin for retaining nitrogen over 14 years span.1972, 1986, 2003 and 2015.While in Cross river basin in 1972, the economic value of nitrogen exported was found to be $23552.30and $15920.30for nitrogen retained with $5260500.00as the economic benefit of the basin for retaining Nitrogen over 14 years span.In 1986 for Imo river basin, the economic value of nitrogen exported was found to be $170820.00and $48844.20 for nitrogen retained with $42610500.00as the economic benefit of the basin for retaining nitrogen over 17 years span For Ikwa Ibo river basin in 1986, the economic value of nitrogen exported was found to be $33798.00and $12923.00for nitrogen retained with $8430760.00as the economic benefit of the basin for retaining nitrogen over 17 years span.For Enyong creek sub basin in 1986, the economic value of nitrogen exported was found to be $115847.00and $30363.80 for nitrogen retained with $28897620.00as the economic benefit of the basin for retaining nitrogen over 17 years span.While in Cross river basin in 1986, the economic value of nitrogen exported was found to be $42767.20 and $22754.70 for nitrogen retained with $10668100.00as the economic benefit of the basin for retaining nitrogen over 17 years span.In 2003 for Imo river basin, the economic value of nitrogen exported was found to be $142825.00and $48844.20 for nitrogen retained with $42610500.00as the value of the basin for retaining nitrogen over 12 years span.For Ikwa Ibo river basin in 2003, the economic value of nitrogen exported was found to be $26329.80and $10633.50for nitrogen retained with $7480480.00as the value of the basin for retaining nitrogen over 12 years span.For Enyong creek sub basin in 2003, the economic value of nitrogen exported was found to be $117377.00and $40502.80 for nitrogen retained with $33347390.00as the value of the basin for retaining nitrogen over 12 years span.While in Cross river basin in 2003, the economic value of nitrogen exported was found to be $39041.90and $21078.00for nitrogen retained with $11092100.00as the economic benefit of the basin for retaining Nitrogen over 12 years span.While in 2015 for Imo river basin, the economic value of nitrogen exported was found to be $294943.00and $100540.00 for nitrogen retained with $65876660.00as the economic benefit of the basin for retaining nitrogen over 12 years span.For Ikwa Ibo river basin in 2015, the economic value of nitrogen exported was found to be $51645.00and $28945.40for nitrogen retained with $11535100.00as the economic benefit of the basin for retaining nitrogen over 12 years span.For Enyong creek sub basin in 2015, the economic benefit of nitrogen exported was found to be $71226.50and $20896.20 for nitrogen retained with $15908700.00as the economic benefit the basin for retaining nitrogen over 12 years span.While in Cross river basin in 2015, the economic value of nitrogen exported was found to be $46501.30and $28405.50for nitrogen retained with 10386200.00as the economic benefit of the basin for retaining nitrogen over 12 years span.
Figure 17 shows the economic benefit of retaining phosphorus in 1972, 1986, 2003 and 2015 in Abia state hydrological basin.Figure 18 graphically shows the economic value of exporting and retaining phosphorus in 1972, 1986, 2003 and 2015 in Abia state hydrological basin.In 1972 for Imo river basin, the economic value of phosphorus exported was found to be $12398.30and $23551.20 for phosphorus retained with $7667290.00as the economic benefit of the basin for retaining phosphorus over 14 years span.For Ikwa Ibo river basin in 1972, the economic value of phosphorus exported was found to be $2711.83and $8408.91 for phosphorus retained with $605699.00 as the economic benefit of the basin for retaining phosphorus over 14 years span.1972, 1986, 2003 and 2015.For Enyong creek sub basin in 1972, the economic benefit of phosphorus exported was found to be $5747.68and $21706.50for phosphorus retained with $1290940.00as the economic benefit of the basin for retaining phosphorus over 14 years span.
While in Cross river basin in 1972, the economic value of phosphorus exported was found to be $5779.82and $15920.30for phosphorus retained with $5260500.00as the economic benefit of the basin for retaining phosphorus over 14 years span.In 1986, for Imo river basin, the economic value of phosphorus exported was found to be $46970.30and $13514.60 for phosphorus retained with $11716530.00as the economic benefit of the basin for retaining phosphorus over 17 years span.For Ikwa Ibo river basin in 1986, the economic value of phosphorus exported was found to be $9476.83and $3584.08 for phosphorus retained with $2363950.00as the economic benefit of the basin for retaining phosphorus over 17 years span.For Enyong creek sub basin in 1986, the economic value of phosphorus exported was found to be $33661.60and $8771.67 for phosphorus retained with $8396750.00as the economic benefit of the basin for retaining phosphorus over 17 years span.While for Cross river basin in 1986, the economic value of phosphorus exported was found to be $11994.80and $6311.25 for phosphorus retained with $2992050.00as the economic benefit of the basin for retaining phosphorus over 17 years span.In 2003 for Imo river basin, the economic value of phosphorus exported was found to be $38255.20 and $23402.00for phosphorus retained with $10868560.00as the economic benefit of landscape for retaining phosphorus over 12 years span.For Ikwa Ibo river basin in 2003, the economic value of phosphorus exported was found to be $7378.98and $3147.89for phosphorus retained with $2096420.00as the economic benefit of the basin for retaining phosphorus over 12 years span.For Enyong creek sub basin in 2003, the economic value of phosphorus exported was found to be $32031.50and $11564.70 for phosphorus retained with $9100352.00as the economic benefit of the basin for retaining phosphorus over 12 years span.In Cross river basin in 2003, the economic value of phosphorus exported was found to be $11214.90and $6168.00 for phosphorus retained with $3186210.00as the economic benefit of the basin for retaining phosphorus over 12 years span.While in 2015 for Imo river basin, the economic value of Water.Environ.Sustainability.3 (2): 1-21, 2023 phosphorus exported was found to be $75646.40and $27838.40for phosphorus retained with $16895930.00as the economic benefit of the basin for retaining phosphorus over 12 years span.For Ikwa Ibo river basin in 2015, the economic value of phosphorus exported was found to be $13469.80and $7938.67 for phosphorus retained with $3008520.00as the economic benefit of the basin for retaining phosphorus over 12 years span.For Enyong creek sub basin in 2015, the economic value of phosphorus exported was found to be $18709.70and $5667.05for phosphorus retained with $4178874.03as the economic benefit of the basin for retaining phosphorus over 12 years span.While in Cross river basin in 2015, the economic value of phosphorus exported was found to be $12313.80and $7521.79 for phosphorus retained with $2750330.00as the economic benefit of the basin for retaining phosphorus over 12 years span.To offset the nutrient lost erosion inflicts on crop production; large quantities of fertilizers are often applied.The economic value of nutrients loss is higher than nutrients gained annually in Abia state between 37.29% to 40.69% in 1972, 20.00% to 37.73% in 1986, 25.65% to 37.11% in 2003 and 22.68% to 37.92% in 2015 for Nitrogen.While for phosphorous, between 75.61% to 78.97% in 1972, 20.67% to 34.48% in 1986, 26.53% to 37.96% in 2003 and 23.25% to 37.92% in 2015.While, it was discovered that replacement strategy is expensive for farmer and usually not affordable by poor farmers.Also, these fertilizers are inputs of fossil-energy; these chemicals can harm human health, affects current water quality standards and pollute the environment.However, the results reveals economic benefits of applying proper conservations through vegetation over the specified time span by identifying areas in the basin where investments in protecting this environmental service will provide the greatest returns.

Abia state Hydrological Basin Resilient Check
In Abia state, a resilient check was performed on the hydrological basin which is known as Abia state basin for 1972, 1986, 2003 and 2015.Figure 19 shows the Abia state hydrological basin resilient check map for 1972, 1986, 2003 and 2015.38.09% of Enyong creek sub basin.The result implies a drastic reduction in the resilient level of the Abia state hydrological basin over time which is low between 1972 and 2015 as well as its agroproductivity, socio economic inequalities, human health and the overall well-being of Abia sate.This highlights the fact that proper conservation measures needs to be applied because this could serve as relief point to improve agro productivity, water quality standard and general well-being of Abia state.

CONCLUSION
The major findings of this study consist of quantifying and mapping the ecosystem service of sediment retention in Abia state basin.InVEST models was able to predict the spatial and temporal patterns of sediment and nutrient loading in Abia state (hydrological) basin.These calibrated models could then simplify future land use planning.Model allowed us to identify land uses, and areas, within the basin that have high potential for erosion, and to quantify the amount of sediment produced from sheet wash annually.This analysis was intuitively spot-checked with our analysis of physical erosion characteristics within the hydrological basin.The analysis made it abundantly clear that land use within the hydrological basin has the potential to significantly impact downstream sedimentation.Another major finding is that the models used from the InVEST software proved useful even for this small scale study, local study and returned relevant and credible results for both land cover modeling and ecosystem services modeling.The economic benefit per basin of filtration by vegetation delivered at the downstream point(s) of interest over the specified time span was analyzed and the economic values of the basin in-terms of gain or loss were properly evaluated in the light of environment sustainability.A resilient check was performed on the hydrological basin and result implies that a drastic reduction in the resilient level of the Abia state hydrological basin was observed over time between 1972 and 2015 which is low as well as its impact on agro-productivity, socio economic inequalities and the overall well-being of Abia sate.This highlights the fact that proper conservation measures needs to be applied because this could serve as relief point to improve agro productivity, water quality and socio-economic standard and the general well-being of Abia state.

Conflict of interest
The author declare that there was no conflict of interest.
thresh = Total allowed annual load for the pollutant of interest; and contrib = Number of pixels on the landscape.Pixel values are then summed or averaged to the basin scale.

Figure 3 .
Figure 3. Minimum, maximum and mean potential soil loss for Abia state hydrological basin in 1972, 1986, 2003 and 2015.
shows the graphical detail of the distribution of nitrogen retained in Abia state hydrological basin for 1972, 1986, 2003 and 2015.Total nitrogen of 23551.20kg/basin in 1972, 48844.20kg/basin in 1986, 84270.50kg/basin in 2003 and 100540.00kg/basin in 2015 was retained for Imo river basin.For Ikwa Ibo river basin, 6172.31kg/basin in 1972, 12923.00kg/basin in 1986, 10633.50kg/basin in 2003 and 28945.40kg/basin in 2015 were retained for nitrogen.For Enyong creek sub basin, total nitrogen of 15920.30kg/basin in 1972, 122754.70kg/basin in 1986, 21078.00kg/basin in 2003 and 28405.50kg/basin in 2015 was retained.For Cross river basin, total nitrogen of 10727.30kg/basin in 1972, 30363.80kg/basin in 1986, 40502.80kg/basin in 2003 and 20896.20 kg/basin in 2015 of nitrogen was retained.
shows the graphical details of the distribution of phosphorous retained in Abia state hydrological basin for 1972, 1986, 2003 and 2015.Total phosphorous of 41555.40kg/basin in 1972, 13514.60kg/basin in 1986, 23402.00kg/basin in 2003 and 27838.40kg/basin in 2015 was retained for Imo river basin.For Ikwa Ibo river basin, total phosphorous of 8408.91kg/basin in 1972, 3584.08kg/basin in 1986, 3147.89kg/basin in 2003 and 7938.67 kg/basin in 2015 was retained.In Enyong creek sub basin, total phosphorous of 21706.50kg/basin in 1972, 6311.25 kg/basin in 1986, 6168.00kg/basin in 2003 and 7521.79kg/basin in 2015 was retained.While for Cross river basin, total phosphorous of 20500.10kg/basin in 1972, 8771.67 kg/basin in 1986, 11564.70 kg/basin in 2003 and 5667.05kg/basin in 2015.The result reveals that eroded soil contains about 3 times more nutrients Abia state Hydrological BasinBased on the Land Use Land Cover (LU/LC) change analysis for each hydrological basin in Abia state presented in figure7nutrient exported and retained were examinedfor 1972, 1986, 2003 and 2015.
Figure15shows the graphical details of the distribution of nitrogen exported in Abia state hydrological basinfor 1972, 1986, 2003 and 2015.

Figure 11 .
Figure 11.Nitrogen exported and retained for Abia state in 1972, 1986, 2003 and 2015.Built up areas was observed to export maximum of 3.01 kg/basin and mean of 0.69 kg/basin of nitrogen in 2003.Bare ground was observed to export maximum nitrogen of 3.07 kg/basin and mean of 0.22kg/basin in 2003.Primary forest was observed to export maximum nitrogen of 0.97 kg/basin in 2003.Wetland was observed to export maximum nitrogen of 2.15 kg/basin and mean of 0.03kg/basin in 2003.Water body was observed to export a maximum of 3.16kg/basin of nitrogen in 2003.While in 2015, for agricultural land nitrogen export was observed to be low with a maximum of 3.13 kg/basin and mean of 0.06kg/basin.Built up areas was observed to export a maximum nitrogen of 3.03 kg/basin and mean of 0.60kg/basin in 2015.Bare ground was observed to export maximum nitrogen of 3.17 kg/basin and mean of 0.21 kg/basin in 2015.Secondary forest was observed to export maximum nitrogen of 3.13kg/basin and mean of 0.01 kg/basin in 2015.Primary forest was observed to export maximum nitrogen of 3.16kg/basin in 2015.Wetland was observed to export maximum nitrogen of 3.12 kg/basin and mean of 0.21kg/basin in 2015.Water body was observed to export a maximum of 3.03kg/basin and mean of 0.07kg/basin of nitrogen in 2015.
forest, maximum of 0.54kg/basin phosphorus was exported in 1986.Primary forest, maximum of 0.36 kg/basin phosphorus was exported in 1986.Wetland, maximum of 0.56kg/basin phosphorus was exported in 1986.Water body was observed to export a maximum of 0.23kg/basin of phosphorus in 1986.In 2003, for agricultural land maximum of 0.61 kg/basin of phosphorus were exported.Built up areas, maximum of 0.68 kg/basin and a mean of 0.16kg/basin of phosphorus were exported in 2003.Bare ground, phosphorus maximum of 0.68kg/basin and mean of 0.06 kg/basin in 2003 was exported.Secondary forest, maximum of 0.26 kg/basin of phosphorus was exported in 2003.Primary forest exported maximum of 0.26kg/basin of phosphorus in 2003 was exported.Wetland exported maximum of 0.55kg/basin of phosphorus in 2003.Water body was observed to export a maximum of 0.48 kg/basin and mean of 0.01kg/basin of phosphorus in 2003.While in 2015, for agricultural land phosphorus export was observed to be low with a maximum of 0.51kg/basin and mean of 0.01kg/basin.Built up areas, maximum of 0.66 kg/basin of phosphorus were exported in 2015.Bare ground, maximum of 0.70kg/basin and mean of 0.19kg/basin of phosphorus were exported in 2015.Secondary forest, maximum of 0.67kg/basin and mean of 0.06kg/basin of phosphorus were exported in 2015.
Figure 20 shows graphical details of the resilience of Abia state hydrological basin for 1972, 1986, 2003 and 2015.The resilient check explains the ability ofthe hydrological basin to response to change in land use in relation to soil loss, sediment export, nutrient export and economic value of nutrient loss.The hydrological basin resilient level has an impact on water quality, agricultural productivity and economy of Abia state.Furthermore in Abia state, hydrological basin resilient check was performed for each basin as shown in Figure 21.In 1972, high resilient covers 18.66% of Imo river basin, 36.12% of Cross river basin, and 45.22% of Ikwa Ibo river basin while low resilient covers 54.31% of Imo river basin, 12.29% of Cross river basin, and 33.41% of Enyong creek sub basin.In 1986, very low resilient covers 44.33% of Cross river basin, and 56.67% of Ikwa Ibo river basin while low resilient covers 48.75% of Imo river basin, 25.24% of Cross river basin and 26.01% of Enyong creek sub basin.

Environ. Sustainability. 3 (2): 1-21, 2023 retained
(Tallis et al., 2013)etained by sub watershed, x; t = time span being considered for the net present value of water treatment; and r = Discount rate used for calculating the net present value(Tallis et al., 2013).The watershed values are then summed to the basin to determine the water purification value per basin.

Results and Discussion Mapping the effect of Sediment Exported and Retained from Soil Loss on Abia state Hydrological Basin Effect of Soil Erosion on Abia state Hydrological Basin
Imo river basin was recorded.Ikwa Ibo river basin recorded a mean soil loss of 55.99 tons/basin, 15.38 tons/basin, 15.54 tons/basin and 19.65 tons/basin for1972, 1986, 2003 and 2015.