Let’s consider that the conditional probability is only proportional to the time interval Δt. This condition is not sufficient because the flow interval, which increases with the time interval, also decreases with time.

If Δt tends toward dt, the solution of Equation (5) is Boussinesq’s equation [1] and Maillet’s equation [2] :

Q₀ is the initial flow at t = 0.

Let’s consider that the conditional probability is proportional to the time interval and inversely proportional to time. This case reflects the behaviour of the recession flow

with. If Δt tends toward dt, the solution of Equation (7) is:

At t = 0, is equal to 1, therefore:

And

If x = 0 and b = 0, Equation (9) gives Maillet’s formula [2] . If b = 0, Equation (9) gives Horton’s formula [4] .

Let’s consider that x = 1 in Equation (7):

If Δt tends toward dt, the solution of Equation (10) is:

As, then C = 0. As, then b = 1. Therefore, the equation of the recession flow is:

Equation (12) is similar to the equation derived by Wittenberg [5] :

With

Equation (13) is related to a power-law reservoir which is suitable for springs, unconfined aquifer and soil moisture.

Equation (12) is similar to Boussinesq’s equation [1] :

With k/a = 2. Equation (14) is suitable for shallow unconfined aquifer.

Equation (12) is similar to the equation of Griffiths and Clausen [9] :

With k/a = 3. Equation (15) is suitable for surface depression storage such as lakes and wetlands.

Equation (12) is similar to:

With k/a =1 and. Equation (17) is related to exponential reservoir and is suitable for modelling throughflow in soil.

Equation (12) is similar to Coutagne’s equation [10] :

With and.

Equation (12) is similar to the following equation related to underground caverns reservoir [9] :

With k/a = ‒1, and

Do these equations reflect a Markovian process or not? In other words, does the future flow depend on the former or not? The answer can be found by the following equation:

By combining Equations (6) and (19), it can be found that for Maillet’s equation:

does not depend on; the process is Markovian.

For Equation (9):

depends on t, therefore on; the process is not Markovian.

For Equation (12):

The process is also not Markovian.

The non-Markovian character of Equations (9) and (13) means that the groundwater storage exhibits a memory effect.

Twelve spring recession flow series (Table 1) are analysed in order to validate the probabilistic approach proposed in this paper. Nine springs are located in USA. The data are provided by the U.S Geological Survey (http://waterdata.usgs.gov/nwis.). Two springs are located in France [11] and one spring is located in ex Montenegro [12] . Rewriting Equation (9) with b = 0 gives:

With and

The computed results of Equations (12) and (23) have been compared with the measured data of the springs. The results are recapitulated in Table 1 and an example of comparison between experimental and computed data

is illustrated by Figure 2.

For each spring, the volume of water released during the recession duration can be computed by:

Equation (24) is a discrete sum. T is the duration of the recession. The volume released can also be computed in a continuous point of view by:

is the Euler gamma function and is the incomplete gamma function.

The Euler gamma function satisfies:

The incomplete gamma function satisfies:

Equation (26) has a limiting value when the duration t is important (mathematically, when the duration t approaches infinity):

Equation (25) also has a limiting value when the duration is important and when λ > 1:

On the other hand, if λ < 1, Equation (25) does not have a limiting value when the duration t is important:

That means if Equation (25) with λ < 1 is used for computing the released volume by the spring and if the recession duration is important, the error made on the calculation of this volume can be large. In this case, Equation (26) is more suitable than Equation (25) although it is a little less powerful to simulate the recession flow.

In a general way, Equation (12) is a little more powerful than the Equation (23) to simulate the recession flows. However, for the calculation of released volume, in the case of a long period of recession, the Equation (26) will have to be used because if the parameter λ of the Equation (12) is less than one, the integral of Equation (12) (Equation (25)), mathematically speaking, does not have limiting value if λ < 1.

It was shown in 1.3 that Equation (13) was similar to the Equation (14) derived by Wittenberg [5] with.

He suggested a value of b = 0.5 for average conditions even if the true value of b is not exactly met. The assumption of b = 0.5 would be more physically based and better fitting for the majority of river basins than the linear reservoir.

Equation (14) is related to the following power-law reservoir:

It is obvious that if λ < 1, Equation (13) can not be related to the power-law reservoir described by Equation (30) because b should be negative.

Table 1 gives the value of λ which is more than one only for Crnojevica spring and Mill spring. For all the other springs, λ is between zero and one. In this case, what is the conceptual model that explains Equation (12)?

The volume released is computed by Equation (25) which can be rewritten as:

If S₀ is the initial volume of groundwater in the aquifer, then, at time t, the groundwater storage is:

A general storage-outflow relation can be expressed by:

With

The conceptual model expressed by Equation (33) explains the recession flows of the springs for which the exponent l of Equation (13) is less than one.