Humidity Absolute Humidity number of grams of water per cubic meter [ML-3] Saturation Humidity maximum amount of water air can hold [ML-3] Relative Humidity = % ratio of absolute humidity to saturation humidity.

Condensation Condensation occurs when air mass can no longer hold all of its humidity. Temperature drops => saturation humidity drops. If absolute humidity remains constant => relative humidity rises. Relative humidity reaches 100% => condensation => Dew point temperature.

Factors affecting Evaporation Water temperature. Air temperature above water layer. Absolute humidity of air above water surface. Wind – keeps absolute humidity low. - may increase the molecular diffusion. Solar radiation – Langley = 1 cal./cm2; SI => joule/m2 = 4.18 x 104 Langleys.

Transpiration Plants pump water from ground to atmosphere; accounts for most vapor losses in land-dominated drainage basin. A function of plant density plant size limited by soil water. Wilting point – surface tension of soil water interface > Osmotic pressure.

Evapotranspiration Evapotranspiration = total water loss due to 1) free water evaporation, 2) plant transpiration, 3) soil moisture evaporation. Potential evapotranspiration – the water loss, which occur if at no time there is a deficiency of water in the soil for the use of vegetation. Actual evapotranspiration.

Limited soil-moisture storage Cool, moist Cool, moist Warm, dry

Ample soil-moisture storage

Formation of Precipitation Humid air mass cooled to dew point temperature. Condensation or freezing nuclei (clay minerals, salt, combustion products …). Droplets coelesce to form raindrops. Raindrops must be large enough such that they do not evaporate.

Precipitation – cont. Adiabatic expansion P decreases => V increases =>T decreases Dry lapse rate – Rising dry air 1OC/100m. Wet lapse rate – Rising moist air 0.5OC/100m.

Effective uniform depth (EUD) of precipitation Arithmetic mean method – the rain gauge network is of uniform density. Isohyetal line method. Thiessen method. - construct polygons - weighted by polygon areas

Events during Precipitation Infiltration capacity (ability of soil to absorb moisture). - varies from soil to soil, from dry to moist. fp = fc + (fo – fc) e-kt fp = infiltration capacity (L/T; ft/s or m/s). fc = equilibrium infiltration capacity. fo = initial infiltration capacity. k = constant (1/T; 1/s) (rate of decreased infiltration capacity)

Initial infiltration capacity Equilibrium infiltration capacity

some water always on the surface All infiltrate some water always on the surface All infiltrate Puddles and overland flow

Water table Water table = undulating surface at which pressure in fluid in pores = atmospheric pressure. Water table

Storm Hydrograph Separate out: overland flow from baseflow. D = A 0.2 Or D = 0.827A 0.2 D = number of days between storm peak and end of overland flow. A = area of drainage basin (miles2; km2)

effluent influent

Effluent => Influent

Rainfall-Runoff Relationships Rational equation: if it rains long enough, the peak discharge from a drainage basin is the average rate of rainfall times the area, reduced by a factor to account for infiltration. Rational equation: peak discharge ~ (rainfall rate) x (drainage basin area).

Rainfall-Runoff Relationships Q = CIA Q = peak runoff rate (L3/T; ft3/s, or m3/s) I = average rainfall intensity (L/T; ft/s; m/s). C = runoff coefficient (dimensionless). A = drainage area (m2 or ft2).

Hydrograph Hydrograph – records discharge rate of a river at a single location.

Stream Gaging Parabolic profile in velocity. Q = VA A = Cross sectional area. qi = vidiwi Q = q1 + q2 + q3 + … + qm m = number of segments, usually 15-30.

Velocity profile in a river

Rating Curve Rating Curve – discharge as a function of stream stage (elevation of the water surface above a datum).

Manning equation V = 1.49 R2/3 S1/2 /n or R2/3 S1/2 /n V – average velocity (L/T; ft/s or m/s). R – hydraulic radius; or ratio of the cross-sectional area of flow in square feet to the wetted perimeter (L; ft or m). S – energy gradient or slope of the water surface. n – the Manning roughness coefficient.

Manning Equation

Duration Curves Percent of time discharge was equal or exceeded. P = 100 m /(n +1) m = serial rank, where 1 = greatest flow. n = number of data values.

low overland and return flows; high baseflow; strong water retaining (unconsolidated sand is thick). High overland and return flows; low baseflow; little water retaining (soils are thin).

Baseflow Recession

Q0

Baseflow recession Q = Q0 e–at Q = flow at time t after recession started (L3/T; ft3/s or m3/s). Q0 = flow at the beginning of recession. a = recession constant (1/T; d-1). t = time since recession began. (T; d)

Recession begins

Determining ground water recharge from baseflow (1) Meyboom method (Seasonal recession method): utilizes stream hydrographs from two or more consecutive years. Assumptions: the catchment area has no dams or other method of streamflow regulation; snowmelt contributes little to the runoff.

Meyboom method Vtp = Q0t1/2.3 Vtp = Volume of total potential groundwater discharge during a complete ground water recession (L3, ft3 or m3). Q0 = baseflow at start of recession (L3/T, ft3/s or m3/s). T1 = time it takes baseflow to go from Q0 to 0.1Q0

Meyboom method – cont. Vt = Vtp/10 (t/t1) Vt - the amount of potential baseflow remaining some time t after the start of recession.

Determining ground water recharge from baseflow (2) Rorabaugh method (Recession curve displacement method): utilizes stream hydrograph during one season.

Increase of Recharge find t1 tc = 0.2144 t1 find QA & QB Vtp = QBt1/2.3 – QAt1/2.3 G = 2 Vtp

Late homework policy Late homework will not be accepted one week after the deadline. Late homework will be penalized: 90% (1 day late), 80% (2 day late), 60% (3 day late), 40 % (4 day late) and 20% (5-7 day late).