Description of Diagnostic Variables in Bathtub Model Output
Statistical Distributions in the Corps of Engineers Reservoir Model Development Dataset
Variable Description Geom. Mean CV Min Max
CONSERVATIVE SUB Conservative substance concentration
user-defined; typically chloride; can be used for verification of water budgets or calibration of longitudinal dispersion rates		        
TOTAL P    MG/M3 Growing-season, mixed layer total phosphorus concentration
Measure of nutrient supply under P-limited conditions		 47.9 0.90 9.9 274
TOTAL N    MG/M3 Growing-season, mixed layer total nitrogen concentration
Measure of nutrient supply under N-limited conditions		 1002 0.64 243 4306
C.NUTRIENT MG/M3 Composite nutrient concentration
Measure of nutrient supply independent of N versus P limitation; equals total P at high N/P ratios		 35.7 0.8 6.63 142
CHL-A      MG/M3 Growing-season, mixed layer chlorophyll-a concentration
Measure of algal standing crop based upon photosynthetic pigment		 9.39 0.77 2 63.6
SECCHI         M Growing-season mean Secchi depth
Measure of water transparency as influenced by algae and nonalgal turbidity		 1.08 0.76 0.19 4.6
ORGANIC N  MG/M3 Growing-season, mixed layer Organic nitrogen concentration
Portion of nitrogen pool in organic forms; generally correlated with chlorophyll a concentration		 474 0.51 186 1510
TP-ORTHO-P MG/M3 Total Phosphorus - Ortho Phosphorus concentration
Phosphorus in organic or particulate forms correlated with chlorophyll a and non-algal turbidity		 30 0.95 4.3 148
HOD-V  MG/M3-DAY Hypolimnetic oxygen depletion rate
Rate of oxygen depletion below thermocline; related to organic supply from settling of  algae, external organic sediment loads, and hypolimnetic depth.  For HOD-V > 100; hypolimnetic oxygen supply depleted within 120 days after onset  of stratification		 77 0.745 36.3 443
MOD-V  MG/M3-DAY Metalimnetic oxygen depletion rate
Rate of oxygen depletion within thermocline; generally more important than HODv in deeper reservoirs (mean hypolimnetic depth >20 m)		 68 0.71 25.1 285.8
ANTILOG PC-1 First principal component of trophic response variables (Chlorophyll a, Secchi, Organic N, Composite Nutrient)
Measure of nutrient supply:
Low:  PC-1 < 50
       low nutrient supply low eutrophication potential
High: PC-1 > 500
       high nutrient supply, high eutrophication potential		 245 1.31 18.39 2460.4
ANTILOG PC-2 Second principal component of trophic response variables
Nutrient association with organic vs. inorganic forms; related to light-limited areal productivity.
Low: PC-2 < 4
      turbidity-dominated, light-limited, low nutrient response
High: PC-2 > 10
      algae-dominated, light unimportant, high nutrient response		 6.43 0.53 1.63 13.39
(N - 150) / P (Total N - 150)/Total P ratio
Indicator of limiting nutrient
Low:  (N-150)/P < 10-12      nitrogen-limited
High: (N-150)/P > 12-15       phosphorus-limited		 17 0.681 4.67 73.3
INORGANIC N / P Inorganic nitrogen/ortho-phosphorus ratio
Indicator of limiting nutrient Low:  N/P < 7-10  nitrogen-limited
High: N/P > 7-10  phosphorus-limited		 29.7 0.99 1.57 127.45
TURBIDITY    1/M Non-Algal turbidity (1/SECCHI - 0.025 x CHL-A)
Inverse Secchi corrected for light extinction by Chl a; Reflects color and/or inorganic suspended solids; Influences algal response to nutrients
Low: Turbidity < 0.4
         allochthonous / non-algal particulates unimportant
         high algal response to nutrients likely
High: Turbidity > 1
         allochthonous particulates possibly important
         algal response to nutrients may be suppressed by light
              limitation and/or nutrient bioavailability		 0.61 0.88 0.13 5.15
ZMIX * TURBIDITY Mixed-layer depth x turbidity
Effect of non-algal turbidity on light intensity in mixed layer
Low: < 3
     light availability high; turbidity unimportant
     high algal response to nutrients
High: > 6
     light availability low; turbidity important
     low algal response to nutrients likely		 3.15 0.78 1.03 17.1
ZMIX / SECCHI Mixed-layer depth/Secchi depth (dimensionless)
Inversely proportional to mean light intensity in mixed layer for a given surface light intensity; reflects combined effects of light extinction by non-algal turbidity and chlorophyll-a.
Low: < 3   
     light availability high
      high algal response to nutrients expected
High: > 6
    light availability low
    low algal response to nutrients expected		 4.77 0.58 1.49 19.0
CHL-A * SECCHI Chlorophyll a x transparency (mg/m2)
Partitioning of light extinction between algae turbidity; Measure of light-limited productivity; Correlated with PC-2 (second principal component)
Low: < 6       
      turbidity-dominated, light-limited
      low nutrient response expected
High: > 16
     algae-dominated, nutrient-limited
     high nutrient response expected		 10.2 0.71 1.79 30.5
CHL-A / TOTAL P Mean Chlorophyll a / Mean Total P
Measure of algal use of phosphorus supply; Related to nitrogen-limited and light-limitation factors
Low: < 0.13
      low phosphorus response
      algae possibly limited by N, light, or flushing rate
High: > 0.40
     high phosphorus response (northern lakes)
     N, light, and flushing unimportant
     P limited (typical of northern lakes)		 0.20 0.64 0.043 0.60
FREQ(CHL-a>10) % Algal Nuisance Frequencies or Bloom Frequencies
Percent of time during growing season that chl-a exceeds bloom criteria of 10, 20, 30, 40, 50, or 60 ppb;  Related to risk or frequency of use impairment. Appropriate bloom criteria may vary with water use, region, & user expectations; Estimated from mean chl-a & assumed temporal coefficient of variation.
FREQ(CHL-a>20) %
FREQ(CHL-a>30) %
FREQ(CHL-a>40) %
FREQ(CHL-a>50) %
FREQ(CHL-a>60) %
CARLSON TSI-P Trophic State Indices (Carlson 1977)
Translates P, Chl a, and Secchi Depths onto a common scale thought to correlate with "trophic state"
Reflects correlations developed from northern lake data sets.
            TSI < 40        “Oligotrophic”
     41 < TSI < 50        “Mesotrophic”
     51 < TSI < 70        “Eutrophic”
            TSI > 70        “Hypereutrophic”		        
CARLSON TSI-CHLA        
CARLSON TSI-SEC